LIASSIC FORMATIONS. 
105 
In all Ichthyosaurs the pectoral limb incl tides a humerus (see the typical restoration, 
PI. XXXIII, fig. I, 53), two antibrachials (54, 55), three proximal carpals (56), and four 
distal ones (o 6 ')> frona 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 (PI. XXVIII, fig. I, 5). 
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 ancono-thenal compression becomes more 
marked and is continued to the distal end, which is pretty ecpially 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 quadrangnlar 
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. I, -56) presents the same relation to the anti- 
brachials as does the proximal row of carpals in Testudo ; ^ 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 metacarjjals (.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 (i, ii, 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 (1') 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 series are present in 
that fin of Ich. communis. 
1 ‘ Anat. of Vertebrates,’ vol. i, p. 174, fig. 108, a, d, c. 
