No. 136.] 103 



ing to the beak. The dorsal valve shows the dental sockets, 

 with a strong thickened process which is deeply concave in 

 the centre from which proceeds the crural processes. 



I refer this to the Genus Centronella of Billings, from its similarity 

 to C. glansfaga ; but it is to be regretted that the figures given to illus- 

 trate the generic character are extremely unsatisfactory. 



Of the Lamellibranchiata of these groups, Mr. Conrad has described 

 about eighty species ; and, including those described by others, we have 

 nearly one hundred known species of these fossils. A considerable 

 number of these remain to be identified. Several species of Gasteropoda 

 have been described, among which are Platyceras sulcatus, P. cirrifor- 

 mis, P. suhundata, P. erecta; Platyostoma lineata; Pleurotomaria poulsoni, 

 P. unisulcata, P. capillaria, P. sulcomarginata, P. rotunda ; Bellerophon 

 unisulcata, B. brevilineatus. 



The following species have not been described: 



MURCHISONIA MAIA (n.s.). 



Shell elongate, turritiform : spire consisting of seven or eight 

 (or more) volutions, which are regularly convex and gradually 

 expanding from the apex, the last volution somewhat more 

 ventricose than the preceding ; aperture broadly oval, a little 

 longer than wide, the columella extended below. Surface 

 marked by transverse threadlike strias, bent backwards on the 

 centre of the volution by an elevated revolving band, which 

 is tricarinate, with depressed lines between. Summit of the 

 volutions canaliculate just below the suture ; the striae bending 

 forward from the suture, and backward as they pass the limit 

 of the groove. Length from one to two inches or more. 



Geological formation. Upper Helderberg limestones. 



MURCHISONIA LEDA ( n. s.). 



Shell elongate, terete or subfusiform : volutions somewhat 

 rapidly expanding, convex, the last two a little more expanded ; 

 aperture elliptical. Surface of volutions marked by elevated 



