-<>' TETRABRANCHTATA. 



If we regard strictly the rules of priority, this >peeies will 

 bear the name of S. (Nautilus) xp/rn/d. Linn.; whilst those of 

 S. prototypUS, IVron. and ti. frayili*. L:nn.. are both antecedent 

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

 well known that to displace it in fa voi- of either of the others 

 would create uncertainty and confusion. It may lie remarked 

 that two pre-Liimean authors perceived its generic distinct ne 

 IVom Nautilus with which Linmeus confounded it ; and one ol 

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

 name of Lituu* adopted. 



ORDER II. TETRABRANCHIATA. 



Animal breathing by two pairs of internal, symmetrical Liills 

 or branchhe. 



Kyes 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 -irdle. Siphon an incomplete tube formed by the union 

 of two lobes. No ink-bain;. Creeping, and protected h\ an ex- 

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



Shell formed of two layers, the external porceljinous. the 

 internal and the septa or part it ions nacreous. Partitions pierced 

 by siphon-tubes. 



Nearly two thousand fossil species of cephalopods have been 

 referred to the tetrabranchiates. although it has been recen'l\ 

 susi)ecte(l that at least a large portion of these were internal 

 shells like the Spirulas and referable therefore to the dibranchiata. 

 Only a half'-do/eii recent species are known; all belono-ino \ ( > 

 the "-enus Nfinlilnn. 



The tet rabranchiate shell is essentially an elongated cone, 

 divided oil' into chambers by partitions, and siphuuciilated. 

 These septa have simply curved ed^es in Nautilus* and Orf/i't- 

 cerOS, they are xi^-xaji' in G<nu'(ilifi'x, or lbli:iceous. forminir com- 

 pli<-ated lobes in Am mo// //<*. The shell may be straight, curved, 

 open or clo>e spii-al. and even vary in form at different HL^'S. and 

 these variations, when well understood, will doubtless cause a 

 reduction to be made in the number of o-eneric forms at 



