158 Geological Society. 



absolute development of the endo- and exo-skeletons in fishes, con- 

 sidered as indications of the perfection of their general organization. 



2. "On a New Species of Plesiosaurus ; with Remarks on the 

 Structure of the Atlas and Axis, and of the Cranium in that genus." 

 By Prof. Huxley, F.R.S., F.G.S. 



The specimen which is the subject of the present paper was pro- 

 cured at Street, near Glastonbury. It is now in the Collection of 

 the Museum of Pi-actical Geology, Jermyn Street, and will be de- 

 scribed at length in the Decades of the Geological Survey. 



It approaches most nearly to P. Hawkinsii ; but the head is 

 smaller in proportion to the body and neck, and the number of the 

 cervical and dorsal vertebrae is different, there being altogether 

 fifty-three cervico-dorsal vertebras, of which thirty are cervical ; 

 while in P. Hawkinsii the cervical vertebrae are thirty-one, and the 

 dorsal at least twenty-three. For this species, characterized by 

 fifty-three cervico-dorsal vertebras, — by a cranium at most not more 

 than -pxth of the length of the bodj', and by having the anterior thirty 

 vertebrae fully, or more than, equal to four lengths of the cranium, 

 the name of P. Etheridgii is proposed. Its dimensions are nearly the 

 same as those of P. Hawkinsii, its length being between 7 and 8 feet. 



By a happy accident the only displacement in the whole length of 

 the vertebral column of this specimen has taken place between the 

 head and the atlas and axis, on the one hand, and between the 

 latter and the third cervical vertebra on the other. By a little 

 careful clearing away of the surrounding parts, it has thus been 

 possible to expose the atlas and axis very easily. They are, as 

 Prof. Owen has stated to be their character in this genus, anchy- 

 losed ; but their structure is totally different from what is seen in the 

 Ichthyosaurus, and closely resembles that of the corresponding parts 

 in the Crocodile. An os odontoideum, very similar to that in the 

 Crocodile, represents, as Rathke long since demonstrated in other 

 Reptilia, the central portion of the body of the atlas ; while its 

 cortical inferior portion and its neural arches form an anterior arti- 

 cular cup for the occipital condyle, as in the Crocodile. 



The author next adverts to the many points of structural corre- 

 spondence observable between Plesiosaurus and Teleosaurus, not only 

 as regards the atlas and axis, but as respects the cranium. 



The existence of a distinct jugal and squamosal, and of a union 

 between the latter and the post-frontal, and the consequent sub- 

 division of the temporal fossa, as in the Crocodile, are indicated. 

 The extension of the exoccipitals and of the pterygoids to the 

 OS quadratum is adverted to ; and the very backward position of the 

 posterior nares ascribed to Plesiosaurus is questioned. Teleologi- 

 cally, such an arrangement appears not very comprehensible : and, 

 on morphological grounds, it is unlikely ; for the posterior nares are 

 more forward on the base of the skull in Gavialis than in Crocodilus, 

 and far more forward in Teleosaurus than in Gavialis. It seems more 

 probable that the so-calledposterior nares of PZes/o5a«/7<5 correspond 

 with the deep fossae on either side of a prominent median ridge 

 visible on the under surface of the basisphenoid of Teleosaurus. 



