722 DE. W. B. CAEPENTER ON THE STEUCTUEE, PHYSIOLOGY, AND 



and which may thus affoi'd valid specific characters. In Antedon rosaceus, and what I 

 believe to be its varietal forms (§§ 6, 7), the second syzygy (Plate XXXVII. fig. 3) 

 occurs at the junction of the 9th and 10th segments, and the third at the junction of 

 the 14th and 15th. 



51. Fifth Brachial. — We now come to that which may be regarded as the ordinary 

 form of the segments constituting the basal portion of the Arm, and which is repeated 

 with great uniformity except where a syzygy intervenes, — the peculiar mode of articu- 

 lation existing between the First and the Second segments not presenting itself else- 

 where. Each articular face (Plate XXXVI. fig. 8, a, b) presents the same disposition 

 of transverse ridges and ligamentous and muscular fossae (a, «, b, b, c, c) ; and the same 

 vertical lamellae for the attachment of the muscles (d, d, figs, a, b, c) are common to both. 

 The articular base of the pinnule is situated at the upper and outer margin of the 

 distal articular surface (fig. b,^); and in this and the succeeding segments (in which its 

 position is alternately on the inner and on the outer side) its presence always enables us 

 at once to distinguish the distal from the proximal face of the segment. These faces are 

 also distinguishable by the much greater inclination presented by the proximal face, the 

 distal face being nearly vertical ; this character, which repeats itself along the entire 

 series of segments, is best seen on looking at the arm from above (Plate XXXVII. 

 fig. 4) or in section (Plate XXXVIII. fig. 11). 



52. Middle Portion.-^As we pass from the basal portion of the Arms in the direction 

 of their termination, we find in the first instance no other change in the conformation of 

 the segments than is brovight about by a considerable diminution of their diameter 

 Avithout a corresponding diminution in their length (Plate XXXVII. fig. 1). Thus 

 in the segment of which the proximal and distal faces and the longer side bearing the 

 pinnule are represented in Plate XXXVIII. fig. 5, A, B, c', we recognize the same arrange- 

 ment of the great transverse ridge, the deep fossa for the elastic ligament between this 

 ridge and the dorsal margin, the pair of shallower fossae for the interarticular ligaments 

 on the ventral side of the ridge, and the larger and deeper muscular fossae chiefly formed 

 by the vertical plates rising up on the ventral margin, with the articular socket for the 

 pinnule encroaching on the muscular fossa of the longer side, that have been described 

 in the last paragraph. But whilst the transverse and vertical diameters of this segment 

 are scarcely more than half those of the Fifth Brachial, the length of the longer side is 

 rather greater than less, so as quite to equal the transverse diameter. With this change 

 of proportions there is an increased obliquity of the articular sm-faces, alike in the trans- 

 verse and in the vertical direction ; and thus the spaces between the lamellae for the 

 attachment of the muscles that pass from segment to segment remain undiminished in 

 length (Plate XXXVII. fig. 2), although those lamellae are brought, by the reduction in 

 the diameters of the segments, so much nearer to the axial canal of the arm. The arti- 

 cular sockets of the pinnules are now quite removed from the articidar surfaces of the 

 segments ; and when two segments are united by syzygy (fig. 1, sg, sg), their conjoint 



' These figures have been inverted by the Artist, so that the dorsal margin is uppermost. 



