336 



THE PORTLAND OOLITE PERIOD. 



CHAP. 



The vertebrae which follow have the same pentagonal outline 

 of the faces, which are deeply concave for a space round the centre, 

 and somewhat flattened toward the edge. Regarded laterally, the 

 upper apophysial cicatrix is large, and tumid, and confluent with 

 the base of the neural spine; the lower one more circular and 

 smaller, both on or near the anterior edge. These characters con- 

 tinue with little change, except in the outline, which gradually 



1 2 3 



Diagram CXX VII.. Ichthyosaurus trigonus. Scale one-fifth of nature. 



i. Last cervical vertebra, seen in front. 2. Seen on the left side. 3. Pre- 



medial dorsal of the same, seen on the left side. 



becomes less pentagonal in figure, till the 22nd, the last of the 

 cervical, which has a united base for the neural spine and dia- 

 pophysis. 



Calling the vertebrae after this point dorsal, we observe the 

 diapophysis to descend continually lower and lower on the middle 

 of the side, and with the parapophysis, which is close to the an- 

 terior edge, forming a projection opposite the widest part of the 

 vertebra, which is somewhat longer and wider in proportion than 

 the bones more in advance (see Diagram CXXVII. fig. 3). 



The 4Oth vertebra has the two apophyses approximate and lower 

 down the side-slope ; the outline of the face narrows upward, and 

 widens downward to a kind of oval, the height being almost equal 

 to the width, and double the length. 



A vertebra which belongs to a point a little farther on, say the 

 45th or 5oth, is of the large diameter of 5*25 inches, with a length 

 of 2*6 : its lateral apophyses are confluent into a sinuous arched 

 cicatrix. 



Still farther on, probably about the 6oth or yoth vertebra, the 

 lateral apophysis is single ; a vertebra having this character is 4-8 

 inches, of equal height to the canal, and 2*25 in length. Others, 

 of smaller size (3-4 across), have a length of 1-4. 



After this the vertebrae, retaining nearly the same figure, become 



