:2()fi TETRABRANCHT.ATA. 



If we regard sti-ictly the rules of pjiority, this species will 

 bear the name of S. (Nautilus) spirula^ Liini. ; whilst those of 

 S. prototyjius^ Peron, and S. fragilis. Lam., are both antecedent 

 to S. Peronii. I allow the latter name to stand because it is so 

 well known that to displace it in faAor of either of the others 

 would create uncertainty and confusion, ft may be remarked 

 that two pre-Linnean authors perceived its generic distinctness 

 from Xautilus with which Linna?us confounded it ; and one ot 

 them, Browne, only pu])lislied a yeai- too soon to have his generic 

 name of Lihnis adoi)ted. 



Order IT. TETRABRANCHIATA. 



Animal breathing by two ]);urs of internal, symmetrical gills, 

 or branchite. 



Eyes pedunculated. Mandibles calcareous. Arms (tentacles) 

 xer}^ numerous, not provided with sucking disks. Body attached 

 to the shell by adductor muscles and the mantle by a continuous 

 horny girdle. Siphon an incomplete tube formed by the union 

 of two lobes. No ink-bag. Creeping, and protected by an ex- 

 ternal concamerated shell, the last chamber of which it inhabits. 



Shell formed of two layers, the external porcelanous, the 

 internal and the septa or partitions nacreous. Partitions pierced 

 by siphon-tubes. 



Nearly two thousand fossil species of cephalopods have been 

 referred to the tetrabranchiates, although it has been recentl}' 

 suspected that at least a large portion of these were internal 

 shells like the Spirulas and referable therefore to the dibranchiata. 

 Only a half-dozen recent s]:)ecies are Ivuown ; all belonging to 

 the genus Nautilus. 



The tetrabranchiate shell is essentially an elongated cone, 

 divided off into chambers by pai'titions, and siphimculated. 

 These septa have simpl}' curved edges in Nautilus and Ortho- 

 ceras, they are zig-zag in Goniatites, or foliaceous, forming com- 

 plicated lobes in Ammonites. The shell may be straight, curved, 

 open or close spiral, and even vary in form at different ages, and 

 these variations, when well understood, will doubtless cause a 

 large reduction to be made in the number of generic forms at 



