4 Watson, New Genera of Upper Liassie Plesiosaurs. 



very slight, and that this lack of knowledge does not 

 depend on lack of material. 



I hope shortly to redescribe all the Whitby reptiles, 

 and the present paper is to be regarded in the light of a 

 preliminary description of two interesting genera. 



The New Genus, Microdcidus. 



This genus is founded for P. homalospondylus, Owen, 

 which is represented by three excellent specimens from 

 the D. commune zone of the Yorkshire coast, and by a 

 fourth not so complete, but entirely free from matrix, 

 from the bifrons zone of Wellingborough, Northampton- 

 shire. The available material is sufficient for a very 

 detailed account of the osteology of the species ; a 

 naturally arranged hind limb being the only part lacking. 



This species has been described by Owen, whose 

 account of the vertebral column is sufficient to characterise 

 it. 



The Manchester Museum specimen, L. 7077, which 

 was collected by W. H. Sutcliffe, E.sq., F.G.S., from Whitby, 

 and presented by him to the Museum, show^s 40 cervicals, 

 5 " pectorals," 17 dorsals, and 15 caudals, the tail not 

 being complete, as the Wellingborough specimen has 22 

 caudals, and that at York 26. The chief characters are : — 



1. The elongation of the centra of the cervicals. 



2. The narrowness of the cervicals across the zygapo- 

 physes, and the fore and aft direction of the latter. 



3. The constricted centra of the dorsal region. 



4. The great height of the neural spines of the dorsals. 



5. The lowness of the centra of the three sacral verte- 

 brae and the great strength of their zygapophyses. 



