basipterygoid processes or occipital fontanelles,' but with well-marked grooves for supra-orbital glands. In Lestris antarcticus at any rate there is a foramen formed round anterior end of supra-orbital groove, as in Chionis (q.v.) In Lestris there is a distinct tendency towards the pseudo-holorhinal condition of the Thinocorida and Glareolidæ.

In Larus the relations of the clavicles, scapula, and coracoids are as in the Charadriidae; but the clavicles provided with a hypocleidium come into nearer relations with the carina sterni, to which they are attached by a ligament. The coracoids are in contact behind the spina (externa) sterni. The same statement may be made of Lestris.

The cervical vertebræ are fifteen in number. The dorsal vertebræ are, as in Alcide and plovers, opisthocoelous. Six (Lestris antarcticus) or seven ribs articulate with the sternum. The first phalanx of the second is commonly perforated.2

As to the pterylosis, the plan is that of the Limicolæ, but the ventral tract does not divide until some way down the neck. The feet are webbed, the hallux is small or absent, and there are twelve rectrices.

The muscular formula of Rissa tridactyla is AX+; of Larus, Stercorarius, and Gygis, AXY+. The other genera have the complete formula ABXY+. Rhynchops has no ambiens.

In Lestris crepidatus and L. antarcticus the semimembranosus is sometimes two-headed, one arising from ischium and one from postacetabular ridge of ilium.

The tensores patagii 3 (see fig. 171) are on the plan of

These are present in the young: see PARKER, Linn. Trans. (2), i. p. 142. ? This does not, however, as it has been stated to do, distinguish the Laridæ from other Limicole; though apparently universal in the gulls (including Anous and Gygis), the same perforations are found in Glareola. The value of this anatomical fact may be judged by the perforation of the same bone in such varied types as Pterocles, Coracopsis obscura, (not in C. nigra), Psittacula passerina, Mach@rhamphus Andersoni, Heliodilus, Caprimulgus, Phaethon candidus (not P. rubricauda), and Fregata minor. These instances are taken from the osteological plates illustrating MM. GRANDIDIER and MILNE-EDWARDS'S Histoire Naturelle de Madagascar.

3 For muscular anatomy of Laridae see GIEBEL, Beiträge zur Anatomie d. Möven,' &c., Zeitschr. f. d. ges. Naturw. x. (1857), p. 20; BEDDARD, A Contribution to the Knowledge of the Anatomy of Rhynchops,' P. Z. S. 1896, p. 299.

those of other limicoline birds, but are apt to be a little more complicated. In Larus argentatus, of which the tendons are represented in the annexed cut, the anterior stronger branch of the brevis tendon gives off a forwards and downwards slip to the extensors of the fore arm, from which arises the usual connection with the longus tendon. There is a second connection between the two tendons.

At the origin of the

B

t.pl

FIG. 171.-TENSORES PATAGII or Rhynchops (AFTER BEDDARD).
t.p.l, tensor longus; t.p.b, tensor brevis; B, wristward slip; F, patagial fan;
A, tendons to ulnar side of arm.

patagial fan is a bony nodule, as in the petrel. The tensor longus tendon also gives off a slip (A, fig. 172) to the inside of fore arm which is also present in the auks (q.v.) In Rhynchops (fig. 171, A) there are two such tendons.

Lestris antarctica, Sterna, and Larus marinus (according to FÜRBRINGER'S figure, Pl. xix.) are much the same, but are without the additional slips A and B. Of these A is present in Rissa tridactyla.

The expansor secundariorum is frequently absent, but it

is present in Larus argentatus, marinus, and glaucus, not in fuscus. It is absent in Sterna and Lestris; present in Anous; absent again in Rhynchops.

The biceps is peculiar in some members of this family. In Larus, Sterna, and Anous the biceps has, as usual, the two heads, humeral and coracoidal; but they form two perfectly distinct muscles, of which one, the coracoidal, soon divides into two distinct muscles again, which are inserted

FIG. 172. TENSORES PATAGII OF Larus argentatus (AFTER BEDDARD
FROM FORBES).

n, osseous nodule. Other letters as in fig. 171.

respectively upon the radius and ulna.

The biceps head

goes to the radius, and as a rule gives off the biceps slip to the patagium; but in Larus ridibundus, according to FÜRBRINGER, this slip arises from the coracoids, an anomaly observable also in the petrel, Thalassiarche. Rhynchops has no biceps slip at all.

The anconaus is generally attached by a tendon to the humerus.

A A

The cæca vary considerably in their development, as the following table of measurements shows:

The lobes of the liver are subequal in Sterna, Anous, Gygis, and Rissa. In Larus and Lestris the right is the larger. A gall bladder is always present.

In Anous the cæca are quite short; in Gygis long and charadriine.

The syrinx of this family is typically tracheo-bronchial (at any rate in Larus), with well-developed muscles, which always reach the bronchi. The family thus differs from other Limicolæ where there is a tendency towards a retrogression of the intrinsic muscles, sometimes culminating in actual disappearance.

In Larus marinus the last six or seven tracheal rings are rather narrower from above downwards than those which precede them, and are more or less firmly attached (except the last ring, which is incomplete both in front and behind) to form a box. When the syrinx is viewed from behind, a broad three-way piece is seen, into the formation of which the penultimate tracheal ring and the four or five in front of it appear to enter. This piece, however, is only really solid at the edges, the bars being a continuation of the penultimate tracheal rings. In the middle it is so thin as to be little more than a membrane. The first bronchial semi-ring (to which the intrinsic muscles are attached) is bow-shaped and in close contact with the last tracheal ring. The remaining semi-rings are narrower and run in a straight direction across the bronchi.

The membrana tympaniformis is distinguished by its

thinness and transparency from the thick yellowish membrane which unites the edges of the greater number of the bronchial semi-rings.

In Larus fuscus and L. glaucus the differences are but slight, and chiefly concern the greater solidity of the threeway piece.

In Larus argentatus the edges of the three-way piece are most solid; but they are connected by a series of four or five bars which divide up the central region of the threeway piece into alternate thicker and thinner portions. This is a peculiar specialisation of the three-way piece which I have not observed elsewhere. There is, furthermore, a thin bony curved rod, closely applied to the outside of the threeway piece, which arises from, or is at least connected with, the last tracheal ring.

Lestris antarcticus has a syrinx which is rather different from that of Larus, and which points in the direction of the Charadriida, owing to the fact that the intrinsic muscles do not reach the bronchial semi-ring except as a fibrous band. The three-way piece, which is solid, is formed by two or three tracheal rings; there is no differentiation in membrane closing bronchial semi-rings.

It is evident from the foregoing account that, while there are a few differences between the various genera of Laridæ, Rhynchops is quite the most anomalous form in structure as well as in external appearance, as seen in its remarkable bill, with its scissor-like edge and projecting mandible. Still, the differences are, in my opinion, not sufficient to place Rhynchops in a family by itself opposite to the remaining Laridæ. It seems that the usually received division of the family, making an additional one for Rhynchops, will serve to divide naturally the Larida; we may term these divisions subfamilies. They will be thus characterised:

Subfamily I. Larinæ.

Muscle formula of the leg, AXY+. Cæca rudimentary. Biceps slips and expansor secundariorum present.