4 FOSSIL REPTILIA OF THE 



the anterior vertebra. The neurapophysial pits now diminish in breadth, pre- 

 serving their length nearly coequal with that of the centrum itself. The terminal 

 articular surfaces show a slight sinuosity, feebly concave, with a less convexity at 

 the middle part. The under surface still retains an aspect of flatness, both from 

 before backward and from side to side. Most of these mid-dorsal vertebras show 

 a slight difference of length in the two sides, as in figs. 9 and 14. 



In one dorsal vertebra (fig. 26) a terminal articular surface, showing a porous or 

 spongy character, is also marked by irregular grooves converging toward the 

 centre, like the corresponding surface of a Cetaceous vertebra from which the 

 epiphysial plate had become attached. Save in this single instance, I never met 

 with such an appearance in a Plesiosaurian vertebra ; the opposite surface is 

 smooth, as are both surfaces in the other vertebras of PI. planus. 



In the tail the broad and short proportions of the vertebral centrum are resumed 

 (figs. 16 — 19), but with a more marked concavity of the terminal articular surfaces, 

 which in one vertebra showed fine lines radiating from the centre. A broad, but 

 almost flattened, border extends from the terminal surface upon the side of the 

 centrum, joining the costal surface, and expanding to mark the place and extent 

 of attachment of the haemapophyses. The diminished size and feebler impression 

 of the neurapophyses bespeak the reduction of the neural arch at this part of the 

 vertebral column. The pleurapophyses retain their independency, and were 

 articulated to a small subcircular surface on the upper half of the side of the 

 centrum ; the lower half is almost flat, and joins, at an open angle, the equally flat, 

 broad under surface, which is bordered, like the sides, by the deflected tract from 

 the articular ends. The venous canals open upon the middle of the under surface, 

 about four lines apart. 



A few small vertebral centrums belonging to the present series, and apparently 

 from a similar-sized Plesiosaur, if not part of the same individual, seem to be 

 reduced to the simplicity of supporting only neurapophyses, and show no distinct 

 marks of articulations for either pleur- or heem- apophyses. The centrums are 

 broad, depressed, with perfectly flat terminal surfaces, and a flattened under 

 surface. They may come from the beginning of the neck, or from the end of the 

 tail. 1 reject the latter notion, because the analogy of the terminal caudal ver- 

 tebrae, or those in which the hasmapophyses ceased to exist, in other Plesiosauri, 

 would lead one to expect a concavity of the articular surfaces, and a diminution 

 in the lateral rather than in the vertical direction, a compressed rather than a 

 depressed form. Assuming, then, that these vertebras are from the beginning of 

 the neck, the question next arises whether pleurapophyses were wholly absent, or 

 whether they were so small and so feebly articulated as to leave no sign of their 

 attachment, at least after the degree, slight as it is, of superficial abrasion to 

 which the fossils have been subject. I think the latter condition may be the more 



