Lamarck's Genera of Shells. 73 



space behind, and remaining stationary for a while in its new 

 position, forms another and a larger chamber. 



This section contains seven families. The shell is multilocular, 

 and the margins of the chambers are simple, without any divided 

 sinuous sutures on the internal surface of the shell. 

 1st Family. 

 Outiioceuata *, (5 genera.) 



Shell straight, or nearly so ; no spiral. 



The shells of this family, as its name denotes, are straight, or 

 only very slightly curved, and consist of an elongated, testaceous 

 envelope, containing a similarly elongated nucleus. When the 

 envelope is solid at the upper part, so that the nucleus does not 

 reach its summit, they are easily separated from each other. The 

 chambers of the nucleus are simple, and generally perforated. 

 Most of the shells of this family are unknown except in the 

 fossil state. 



1. Belemnites \. 



Shell straight, elongated-conical, formed of two distinct and 

 separable parts ; viz. 



The external, a solid sheath, full at the upper part, with a 

 conical cavity ; 



The internal, a conical nucleus, pointed, chambered transversely 

 through its whole length, multilocular; chambers slightly concave 

 on one side, and convex on the other, and perforated by a central 

 siphon. 



The belemnites, which are only found fossil, and generally 

 empty, or without the nucleus, are merely the sheath of an 

 elongated-conical mass, not adhering, chambered, and furnished 

 with a siphon like the orthocera and hippurites. The form of the 

 sheath is that of a long cone, more or less pointed at the summit, 

 and it often has a shallow lateral groove ; its upper part is solid, 

 whilst the lower has a conical cavity, which contains the multilo- 

 cular nucleus. 



Type. Belemnites subcorneal. (Nautilus belemnita. Gmcl.) 



I From ofio;, straight, ami xt 5 -a;, a horn. 

 t From Sty.s;, untie /Ssas^wv;'/, a dart. { Subeonicot. 



