74 7 he Irish Naturalist. [ March, 



tusk {Dentalitim, e.g.), and is open at both ends. Deiitalhtm is a 

 tolerably common fossil in the Carboniferous limestone, but 

 it is difficult to extract it from the rock except in fragments. 



The shell in the Gastropods is typically conical in outline, 

 consisting of a spirally coiled, expanding tube, or simply of a 

 more or less elevated cone, as in the full-grown limpet ; the 

 young has a spiral shell. Perforations, or slits in the shell, of 

 classificatory importance both in recent and fossil shells, 

 indicate corresponding structures in the mantle-fold. These 

 are met with in Pleurotomaria, Haliotis (the common " ear- 

 shell "), and in Fissurella, the " key-hole " limpet. 



A notable exception to the ordinary plan of the Gastropod 

 shell is encountered in the Chiton (" coat-of-mail " shell) 

 whose shell consists of eight flattened and slightly arched 

 transversely oblong valves, partly or entirely embedded in 

 the mantle. Fossil Chitons are found in the Carboniferous 

 Limestone of Belgium and Ireland. 



Lastry, in the Cephalopoda there is only one living member 

 of the class, that is Nautilus, that has a complete external 

 shell, secreted by the mantle. An imperfect form of this 

 kind of shell, consisting only of the pearly layer, exists in 

 a small dibranchiate cuttle-fish called Spirilla ; but in this 

 the shell is practically internal. 



The structure of the shell in Nautilus (a tetrabranchiate or 

 four-gilled Cephalopod) merits a little more explanation, as it 

 elucidates that of a very large group of fossil Cephalopods 

 which includes the Ammonites of the Mesozoic rocks and 

 their predecessors, and the Goniatites of the Devonian and the 

 Carboniferous rocks. In the latter the Goniatites are common 

 in Ireland. The shell of Natitilus is a rapidly expanding 

 cone, spirally coiled, with the coils touching one another, all 

 but the outer coil being generally hidden by the succeeding 

 ones. There is a minute perforation at the commencement, 

 where the first turn took place, which, however, in some fossil 

 forms is very large owing to the wider curve described by the 

 first whorl and the slight amount of overlapping of the 

 succeeding ones. Stretching across the shell at frequent 

 intervals is a series of arched transverse partitions or septa, 

 dividing the shell into chambers, of which the last and 

 largest is occupied by the animal, and is hence called the 



