8 OPTHALMOSAURUS PLEYDELLI. 



the eye and not at the distal end of the snout. The head is very 

 large and prolonged into a more or less elongated snout, the brain 

 cavity is remarkably small, and the eye large. The orbit of I.platyodon, 

 the largest known species, has been found to measure 14 inches in 

 its long-diameter. The enormous size of the eye must have 

 admitted a large amount of light, and have enlarged its powers of 

 vision ; by the muscular adjustment of the sclerotic plates by which 

 objects far and near would be readily focussed and clearly depicted 

 upon the retina to aid the animal in pursuit of its prey. The eye 

 of turtles, lizards, and rapacious birds of the present day are also 

 furnished with sclerotic plates. Birds of prey, which soar to great 

 heights, can command by means of these plates a uniform and 

 continuous change of focus in their descent upon their victims 

 below. They aid equally nocturnal birds of prey by admitting 

 a larger amount of light when the rays at eventide are few and 

 feeble. As this family has no sacrum, the vertebral column 

 consists of a precaudal and a caudal series only ; the centra of 

 the precaudals have a pair of tubercles for the articulation of the 

 double-headed ribs, whilst in the latter the tubercles coalesce. 

 In the cervical region they are placed near the neural-arch, on 

 the lateral surface of the centrum and gradually descend, towards 

 the inferior part, reaching the base where the precaudals terminate. 

 The shoulder-girdle of the Ichthyosaurus is furnished with a clavicle 

 and inter-clavicle, which in Ichthyosaurus proper is T shaped. 

 It was upon these elements and the three-faceted humerus, which 

 differs materially from the original type, Professor Seeley founded 

 his new genus Ophthalmosaurus. The anterior and upper surfaces 

 of the clavicles, which are massive, are united by a suture, and 

 instead of the transverse wedge of the inter-clavicle lying beneath 

 the clavicles, by which it is partly embraced, it is fitted into a deep 

 groove which occupies about five inches of the median portion, and 

 separates the two bones by about an inch ; this again is a divergence 

 from the Ichthyosaurian type, which has united clavicles. The 

 clavicles of Ophthalmosaurus form a crescent-shaped bone, the horns 

 having a backward direction. The concave or posterior surface is 



