24 Watson, Neiv Genera of Upper Liassic Plesiosaurs. 



Note added Dec. yth, 1909. 



Dames in 1895 described an excellent skeleton of a 

 Plesiosaur from the Upper Lias of Germany. This species 

 Plesiosaurus Guilelnii Imperatoris^ he refers to Lyddeker's typical 

 group of the genus Plesiosaurus, i.e., to Plesiosaurus sensu strictu. 



From his description and figures I much doubt the propriety 

 of this reference ; the animal presents many striking resemblances 

 to Microcleiilus ; although it certainly cannot be included in that 

 genus. 



The resemblances are as follows : — 



1. The head of P. Gidlelmi Imperatoris is small, and, 

 although damaged, appears to closely resemble that of Micro- 

 cleidus. 



2. Both species have long necks. Microdeidus having 40 

 cervicals, the other species 35. 



3. The shape of the posterior cervicals shewn in Dames' 

 small but very clear figure is very similar to that of similiarly 

 situated bones in the British genus. 



4. The very high neural spines of the dorsal vertebrae of 

 P. Guilelmi Imperatoris are unparalleled in Plesiosauriis 

 sensu strictu, and are only to be matched in Microdeidus. 



This character is, however, less pronounced in the German 

 species, in which, however, the neural spines of the posterior 

 cervicals are relatively longer than in Microdeidus. 



5. The constricted dorsal centra are also similar in the two 

 species. The two types, however, differ in the following 

 characters : — 



The cervical centra of the P. Guileifni Imperatoris are 

 shorter than those of Microdeidus. It is, however, noticeable 

 that the anterior cervicals of the latter genus are not much 

 elongated, the 6th or 7th being the first in which this character 

 becomes conspicuous. In the pectoral girdle of Dames' 

 specimen the scapulae do not meet in the middle line, and there 

 is a " furculum." Of this furculum only the right half is pre- 

 served, it shews no trace of an interclavicle, and Dames suggests 

 that the latter bone may have been connected by loose suture. 



