LTASSIC FORMATIONS. 105 



In all Ichthyosaurs the pectoral limb includes a humerus (see the typical restoration, 

 PI. XXXIII, fig. 1, 53), two antibrachials (54, 55), three proximal carpals (5G), and four 

 distal ones (56'), from which the more numerous series of ossicles (57) are continued. I 

 shall here limit the description of this part of the skeleton to the modification presented 

 in the Ichthyosaurus communis (PL XXVIII, fig. 1, s ). 



In this species the length of the humerus is but one fourth more than its breadth, 

 and this is greater at the proximal than the distal end. The joint-surface of the head of 

 the bone is subconvex, produced outwardly or anconally upon a thick deltoid ridge, 

 subsiding half way down the shaft ; there the ancouo-thenal compression becomes more 

 marked and is continued to the distal end, which is pretty equally divided into two sub- 

 concave, almost flattened, surfaces for ligamentous union with the antibrachials. 



Assuming the prone position of the fin, which presents to outside view its anconal 

 surface, as in Fishes, the anterior antibrachial represents the radius (54), the posterior one 

 the ulna (55)- Both bones are pentagonal by reason of the truncation of their distal 

 approximated angles, which give lodgment to the proximal angle of the middle hexagonal 

 carpal bone ; the radial and ulnar carpals are transversely oblong, and the quadrangular 

 shape is but slightly disturbed by the production of their contiguous borders into the 

 intervals between the midcarpal and the two metacarpals, which it partly supports. The 

 radial and ulnar ossicles of this third series from the humerus are extended transversely ; 

 the four of the following series articulate each with its corresponding metacarpal. 



The series of three bones (PI. XXXIII, fig. 1, 56) presents the same relation to the anti- 

 brachials as does the proximal row of carpals in Testudo ; l and the series of four ossicles 

 which follows might be homologised with the distal series of carpals in the same number 

 in Testudo. In this case the next transverse row of four ossicles, the third from the 

 antibrachium, may be regarded as metacarpals (57). According to this view the radial 

 metacarpal (57), not the ulnar one (57') supports two digits, and the normal digits in 

 Ichthyosaurus communis are thus five in number (1, 11, in, iv, v). Each consists of a 

 series of flattened, somewhat transversely extended ossicles, of which I have counted thirty 

 and upwards in the two ulnar digits of the present species ; they are rather fewer in the 

 two radial and the mid-digits. But, in addition to these multiplied digital joints there 

 are two superadded marginal series of ossicles ; that (i') on the radial border of the fin 

 begins between the second and third joints of the radial digit, and is continued to near 

 its extremity. The series (v') along the opposite, ulnar, margin, begins at the interval 

 between the ulnar proximal and distal carpals, and is also continued to near the extremity 

 of the fifth normal digit. These supplementary ossicles are more rounded in shape than 

 the normal phalanges, but, like these, progressively decrease in size to the tapering end 

 of the fin. At first view, apart from the preceding homological analysis of the bones of 

 the fore limb of Ichthyosaurus, they seem to show that seven digital scries are present in 

 that fin of Ich. communis. 



1 ' Ar.at. of Vertebrates,' vol. i, p. 174, fig. 108, a, d, c. 



