LIASSIC ICHTHYOSAURS. 61 



advanced than in Ichthyosaurus. The Crocodilia depart widely from the Ichthyosauri 

 in their single and backwardly placed palatal nostril. 



The 'foramen parietale ' (Tab. XIX, tig. 1,/) receives a much smaller proportion, if 

 any, of the parietal bones than of the frontals in its formation (Tab. XXII, fig. 1). 



The upper outlet of the temporal fossa (Tabs. XIX and XXIII, fig. 1,t) is bounded 

 mesially by the parietal, laterally by the mastoid and postfrontal," behind by the mastoid 

 and parietal, in front by the postfrontal ; the lateral or outer wall of the fossa is formed 

 by the mastoid, postfrontal, prosquamosal, postorbital, malar, and zygomatic bones. 



More or less of the circle of sclerotic |)lates are commonly preserved in the fossil 

 skulls of Ichthyosauri. They are of an irregular, olilong, quadrate form, joined together 

 by squamous or overlapping sutures at their longest sides. The hind part is usually 

 about half the length of the plate, and is very thin, ending in a trenchant border; the 

 front or pupillary corneal border is thicker, shorter, and nearly straight. From this 

 border each plate extends, raylike, outward, for more than half its length, then suddenly 

 bends towards the back of the eyeball, defining and encasing its periphery, and indicating 

 the extreme oblateness of that visual spheroid. In the Ichthyosaurus communis I have 

 counted seventeen of these sclerotic plates. 



c. Pectoral and Pelvic Arches and Appendages. — The limbs of Ichthyopterygia, as the 

 name of the order implies, resemble the fins of Fishes in the number of digital joints or 

 segments, and, in some species, the seeming excess of digits beyond the typical ' five.' 

 With the parial ones of Fishes these Reptilian fins also correspond, the anterior pair with 

 the ' pectorals,' the posterior pair with the ' ventrals.' 



The inverted arch^ supporting the ' pectorals ' is detached from the occiput, as in the 

 Plagiostomes ; that supporting the ' ventrals ' is also detached from the ' sacrum,' but 

 retains the position beneath the vertebrae, which, when coalesced, receive that anthro- 

 potomical name. The hinder arch^ has gained a structure determinative of the homology 

 with the hsemovertebral elements called ' pelvis,' and the limbs so supported are called 

 ' pelvic' 



The pectoral arch (Tab. XXIV, fig. 4) consists of a pair of scapulae (51), a pair of 

 coracoids (52), a pair of clavicles (08), and an episternum (-1&). In some specimens there 

 appears a trace of a pair of precoracoids. 



The correspondence with the same arch in Ornithoryhnchus was pointed out and 

 figured by Clift.^ I have not seen an Ichthyosaurus in which the clavicles were 

 confluent mesially as a single bony arch, resembling the Avian ' furculum ; ' but such 

 confluence does take place in the full-grown or aged Monotremes. No sternebers succeed 

 the episternum in Ichthyosaurus as they do in Ornithorhynchus. 



The episternum is small ; each clavicle exceeds the length of the anterior transverse 



1 The definition of ' girdle ' in our Dictionaries is inapplicable to these parts of the skeleton. 



2 'Philos. Trans.,' mdcccxviii, p. 32, pi. ii. 



