104 
POSSIL REPTILIA OF THE 
The inverted arclT supporting the ‘ pectorals ’ is detached from the occiput, as in the 
PJagiostomes ; that supporting the ‘ ventrals ’ is also detached from the ‘ sacrum,’ but 
retains the position beneath the vertebrse, which, when coalesced, receive that anthro- 
potomical name. The hinder arcld has gained a structure determinative of the liomology 
with the haemovertebral elements called ‘ pelvis,’ and the limbs so supported are called 
‘ pelvic/ 
The pectoral arch (Plate XXVIII, fig. 4 ) consists of a pair of schpulee (51), a pair of 
coracoids (52), a pair of clavicles (58), and an episternum (46). In some specimens there 
appears a trace of a pair of precoracoids. 
The correspondence with the same arch in Ornitlioryhiclms was pointed out and 
figured by Cliet/ I have not seen an Ichthyosaurus in which the clavicles were 
confluent mesially as a single bony arch, resembling the Avian ' furculnm ; ’ but such 
confluence does take place in the full-grown or aged Monotremes. No sternebers succeed 
the episternum in Ichthyosaurus as they do in Ornithorhynclms. 
The episternum is small; each clavicle exceeds the length of the anterior transverse 
ray ; the medial longitudinal ray or stem does not exceed the transverse portion in extent. 
The clavicles are powerful bones, pointed at each end, overlying the transverse rays of 
the episternum, and continued along the anterior border of the scapulm towards or near 
to the 'base’ or free extremity of those bones; the joints are rough or sutural. The 
scapulae are oblong, subcompressed, truncate at the free or basal end, thickened and 
broadened at the opposite or articular end for the two joint-surfaces of the coracoid 
and humerus (53) respectively. 
The two pairs of limbs (ib., fig. 1,5, ^>) have been found in every sufficiently preserved 
skeleton, and where such fins have been lost their supporting arches or some elements 
thereof have usually indicated their existence. Of these limbs the anterior or pectoral (5) 
surpass in size, but in different degrees according to the species, the posterior or pelvic {p) 
pair. They appear to be most nearly equal in size in the skeleton, in part restored, of the 
Ichthyosaurus platyodon (PI. XXXI, fig. 1 ), but confirmatory evidence of the degree of 
difference is desirable in regard to this species. The pelvic pair is the smallest relatively 
in Ichthyosaurus latimanus, Ich. communis, and Ich. breviceps, but the inferiority is 
nearly the same in Ich. intermedins (PI. XXX, fig. 1 ). 
In all the species the digits are supported by flattened, subquadrate, hexangular, 
pentangular, transversely quadrate, or rounded phalanges, exceeding in number in each 
digit that known in any other Reptile, and recalling the many -jointed rays of the pectorals 
and ventrals of Fishes. 
The shorter-snouted species have the greater number of digits, with more and smaller 
pnalanges ; as the jaws proportionately elongate the number of digits decrease, and their 
phalanges become relatively larger and fewer. 
^ The definition of ' girdle ’ in our Dictionaries is inapplicable to these parts of the skeleton. 
2 ‘ Philos. Trans.,’ MDCccxvm, p. 32, pi. ii. 
