Mr. L. Barrett on the Atlas and Axis of the Plesiosaurus. 361 



mere local varieties are now deemed what are called "good spe- 

 cies." Dr. Hooker and other botanists have shown how very 

 widely distributed many of our common plants are. On another 

 occasion I may enter on this subject. — A. W. 



Besides these, there are the two following species of the ex- 

 tensive group of the Calandridae, the former of which is very 

 distinctly coloured with two transverse, yellowish- red, subinter- 

 rupted bands on the elytra : — 



Sphenophorus Spinolce, Sch. Gen. et Sp. Curcul. iv. p. 883 

 (1838). 



Scyphophorus inter stitialis, Sch. he. cit. p. 856. 



XXXIX. — On the Atlas and Axis of the Plesiosaurus. 

 By Lucas Barrett, F.G.S*. 



[With a Plate.] 



In a young specimen of Plesiosaurus presented to the Geological 

 Museum of the University of Cambridge by Thomas Hawkins, 

 Esq., the atlas and axis have not coalesced, and are detached 

 from the remainder of the cervical series. The axis is nearly 

 entire ; but the atlas has lost part of its posterior articular sur- 

 face, and the whole of the second subvertebral wedge-bone. The 

 interesting unanchylosed condition of the four bones composing 

 the atlantal cup is a sufficient excuse for occupying a small 

 portion of the time of the Section with a comparison of these 

 bones with those described by Prof. Owen in the 'Annals of Na- 

 tural History ' for 1847, and the corresponding parts of the ske- 

 leton of the new species of Plesiosaurus described by Prof. Huxley 

 in the last Number of the ' Geological Journal/ 



We will first consider the structure of this specimen (PI. XIII. 

 figs. 1 & 2). The four bones composing the atlantal cup have been 

 slightly displaced ; and its shape is a little altered. The base of 

 the neural canal is formed anteriorly partly by its centrum and 

 partly by the expanded bases of the neurapophyses ; posteriorly 

 the centrum is much larger, and forms the entire base of the 

 canal. The upper thirds of the neurapophyses are much ex- 

 panded and bent backwards, their inner angles have not coalesced, 

 and there is no trace of a neural spine. The anterior surfaces 

 of the lower part of the neurapophyses are concave, and form 

 the antero-lateral segments of the articular cup for the occipital 

 condyle ; laterally their inferior edges slightly overlap the first 

 wedge-bone ; posteriorly they thin away, exposing the postero- 

 lateral edges of the centrum (fig. 1, c). 



* Communicated by the Author, having been read at the Meeting of the 

 British Association at Leeds. 



