CEPHALOPODA. Al 
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. 
Orper—TETRABRANCHIATA. Owen. 
Family—NavtTiLiv&. 
According to Von Buch, the division, which has been made of the tetrabranchiate 
Cephalopods into the two great families Nautilide and Ammonitide, 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 Wautilide, 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 Wawtilide, are characterised by simple curvatures or 
undulations, and by having their margins entire; while, among the dmmonitide, 
the septa present a series of lobes or sinuous flexures, the margins of which are 
foliated. 
A third group, however, exists, m 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 Mister established the genus C/ymenia, has been hitherto generally associated 
with the Nautilidz ; but I propose to separate it as a distinct family, under the name 
Clymenide. 
The Nautilide 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 Nautilide will consist of the following genera: Nautilus, 
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