20G TETRABRANOHIATA. 



If we regard strictly the rules of priority, this species will 

 bear the name of S. (Nautilus) spirula, Linn. ; whilst those of 

 S. prototypus, 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 favor of either of the others 

 would create uncertainty and confusion. It may be remarked 

 that two pre-Linnean authors perceived its generic distinctness 

 from Xautilus with which Linnaeus confounded it ; and one of 

 them. Browne, only published a year too soon to have his generic 

 name of Lituun adopted. 



ORDER II. TETRABRANCHIATA. 



Animal breathing 1 bv two pairs of internal, symmetrical "ills 

 or branclmv. 



Eyes pedunculated. Mandibles calcareous. Arms (tentacles) 

 very 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 recently 

 suspected that at least a large portion of these were 1 internal 

 shells like the Spittllas and referable therefore to the dibranehiata. 

 Only a half-doxen recent species are known ; all belonging to 

 the genus Nautil-H*. 



The tetrabranchiate shell is essentially an elongated cone, 

 divided off into chambers by partitions, and siplmneulated. 

 These septa have simply curved edges in Nanfiln.s and Ortlio- 

 aeras. they are zig-zag in Gonwtifcx, or foliaeeous. forming com- 

 plicated lobes in Ammomlex. 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 



