XIII. 



IGHTHYOSA UEUS. 



337 



shorter, and often deeply depressed in the middle region; lastly, 

 the lateral apophysis is lost, and small, short, somewhat oval 

 excavated plates continue the series to the end, where the vertebrae 

 again become relatively longer. 



Diagram CXXV1II. Anterior caudal vertebrae of Ichthyosaurus trigonus. Scale 



one-fifth of nature, 

 i. Seen in front. 2, 3. Seen on the left side. 



ICHTHYOSAURUS THYEEOSPONDYLUS. Owen. 



This species occurs at several points in the Kimmeridge clay near 

 Oxford, as Winslow, Nuneham, Shotover, Wheatley; and it is found 

 at Weymouth. Its most obvious character is the general shortness 

 of the vertebrae, whose length is on an average less than two-fifths 

 of the diameter. It is usually found to be of smaller size than 

 other kindred remains, the largest known not exceeding 3-5 inches 

 in diameter. 



The cervical vertebrae of this group have the general form and 

 structure of those of I. trigonus ; but there are no examples of the 



Diagram CXXIX. Ichthyosaurus thyreospondylua. Scale one-fifth of nature. 



I. Cervical vertebra, seen in front ; the zygapophyses almost vertical. 2. Seen 



on the left side. 3. Seen from behind. 



first few vertebrae. An approach to pentagonal figure does however 



