338 



THE PORTLAND OOLITE PERIOD. 



CHAP. 



occur, and one has the space round the centre of the disk deeply 

 hollowed. They are very much shorter, remarkably so in the lower 

 part, as if the neck were concave downwards. Length, ro inch; 

 breadth, 3*7 ; height to the canal, 2,' 5. 



Anterior dorsals presenting a nearly circular excavated disk, with 

 prominent lateral cicatrices, and similar proportions as to thickness, 

 are followed by middle and posterior dorsals of similar characters. 



123 4 



Diagram CXXX. Ichthyosaurus thyreospondylus. Scale one-fifth of nature. 



i. Anterior dorsal vertebra, seen in front. 2. Seen on the left side. 3. Pos- 

 terior dorsal, seen on the left side. 4. Seen in front. 



Some of these are shorter in proportion than the others, the length 

 being less than one inch to a breadth of 2*75. The last dorsals 

 and the first caudals are contracted in breadth above, and enlarged 

 below, as in I. trigonus ; they are higher than broad, and have 



Diagram CXXXI. Ichthyosaurus thyreospondylus. Scale one-fifth of nature. 

 I. Caudal vertebra, seen in front. 2. Seen on the left side. 



only small lateral cicatrices. Beyond this we have others of like 

 form, but shorter, without any lateral cicatrix, and perforated in 

 the centre. The length of these is only one-fourth of the diameter. 

 It is probable that no further important variation occurred in the 

 smaller end toward the extreme part of the tail. 



