CEPHALOPODA. 41 



This shell is oblong, regularly convex on the upper surface, and terminated by a 

 very obtuse, short umbo, compressed laterally, and slightly inflected towards the ventral 

 aspect. The ventral margins are depressed, and present outwardly sharp edges, 

 which extend rather more than one third of the length of the shell ; the margins 

 assume a convex form as they approach the inferior extremity, and at about two thirds 

 of the length, become and continue nearly parallel until their union above the um- 

 bonal pore. The inner edges present three obscure, very oblique folds, from which 

 character the specific name is taken. The umbonal pore is circular, and extends to 

 the pyrites, with which the phragmocone is filled ; it is about one fourth of the breadth 

 of the shell in diameter. The septa are distant. 



This unique and valuable specimen enriches the cabinet of Mr. Sowerby, whose 

 kindness in conceding the use of it for description I beg to acknowledge. It was 

 found in the clay removed in constructing the archway at Highway. 



The length is "5 in. ; the breadth at the upper extremity is '25 in., and across the 

 umbonal pore '15 in. 



O&vek—TETRABRANCHIATA. Owen. 

 Family — Nautilid^e. 



According to Von Buch, the division, which has been made of the tetrabranchiate 

 Cephalopods into the two great families Nautilida and Ammonitida, has been determined 

 solely by the position of the siphuncle, which, in the latter family, is invariably placed 

 on the ventral margins of the septa ; while, among the Nautilida, it is placed at or near 

 the centre of the discs of the septa. Other differences exist in the form and condition 

 of the septa, which, among the NautUida, are characterised by simple curvatures or 

 undulations, and by having their margins entire ; while, among the Ammonitida, 

 the septa present a series of lobes or sinuous flexures, the margins of which are 

 foliated. 



A third group, however, exists, in which the siphuncle is placed on the dorsal 

 margin, and the septa are distinguished by angular or rounded lateral lobes, but 

 their margins are perfectly simple. This group, for the typical forms of which 

 Count Miinster established the genus Clt/menia, has been hitherto generally associated 

 with the Nautilidae ; but I propose to separate it as a distinct family, under the name 

 Clymenida. 



The NautUida will then be confined to those genera in which the siphuncle is 

 central or excentric, that is, placed at the centre of the disc of the septum, or between 

 that and the margin ; or, more strictly, to those in which it is not placed either on the 

 ventral or on the dorsal margin. 



As thus restricted, the Nautilidae will consist of the following genera : Nautilus, 



•6 



