92 Oeological Society. 



character but in ever}' other, SipTionia was shown to approach Disco- 

 dermia so closely as to be almost identical with it. 



The mineral replacements which have affected the siliceous 

 skeleton of Siphonia were then considered : in specimens preserved 

 in phosphate of lime from the Gault of Folkestone the spicules 

 have undergone a replacement by calcic carbonate, while those from 

 the greensand of Haldon and other localities still possess a siliceous 

 composition though the interior of the spicules has been dissolved 

 away so as to enlarge the axial quadriradiate canal to a surprising 

 extent ; and the silica so dissolved has been re-deposited on the 

 exterior of the spicule, so as to fill up the interstices of the net- 

 work, and in some cases the cavities of the canal-system of the 

 sponge. Thus, to some slight extent, these specimens may be said 

 to have fossilized themselves. 



Choanites was shown to be the deep-sea form of Siphonia, the 

 latter characterizing (ireensand deposits which were laid down 

 in depths corresponding to those in which existing Lithistids now 

 flourish, while the former is characteristic of the Chalk which was 

 deposited in a deeper sea. 



The paper concluded with a systematic description of the genus. 



June 6th, 1S77.— Prof. P. Martin Duncan, M.B., P.ll.S., 



President, in the Chair. 



The following communications were read : — 



1. "On the Eank and Affinities in the Reptilian Class of the il/o- 

 f^asauridfe, Gervais." By Prof. K. Owen, C.B., r.ll.S., F.G.S. 



The author stated that while the Mosasaurians had been origi- 

 nally referred to the Cetacea by Camper, then to Crocodilia by Faujas 

 de St. Fond, and to the Lacertilia by Cuvier, Prof. Cope had recently 

 thought he recognized in them Ophidian affinities, spoken of them 

 as " sea-serpents," and formed of them an order called Pythono- 

 morpha. He then discussed in detail the various characters presented 

 by the remains of these animals, and arrived at the following con- 

 clusions : — In tbe single occipital condyle and the composite struc- 

 ture of the mandible the Mosasaurians are Reptilian, as also in their 

 procoelian vertebra} ; in the double occipital hypapophyses, the 

 bifurcate and perforate parietal, the presence of the " columella," 

 the composite formation of the suspensory joint of the tympanic and 

 in the type of the tympanic, the frame of the parial nostrils, and the 

 structure and attachment of the teeth they are Lacertian. In one 

 special dental modification they are Iguani an, in another Monitorial; 

 and their special group characters consist in the more extensive fixa- 

 tion of the pterygoids and ossification of the roof of the mouth, the 

 large proportion of the vertebral column devoid of zygapophyses, the 

 confluence of the haemal arch with the centrum in certain of the 

 caudal vertebrae,' and the natatory character of the fore and hind 

 limbs. These distinctive characters did not appear to the author to 

 be sufficient for ordinal rank, and with P. Gervais he regarded the 



