458 



THE GEOLOGIST. 



genus ; and it was this which led Professor M'Coy to designate the group 

 by the name of Athyris, the etymology of which would imply a shell 

 without a door, or, in more appropriate words, without an opening."^ 

 It is, however, certain that the larger number of species and specimens 

 exhibit a clearly truncated beak and circular opening, so that all that 

 can be inferred is, that the animal could, at some time of its existence, 

 or from peculiar causes, dispense with its mooring, and that this was 

 caused by the beak becoming so much incurved or adpressed as to leave 

 hardly any space for the passage of peduncular fibres. 



In the interior of the large valve the hinge- teeth are supported by 

 vertical shelly plates, and the free space on the bottom of the valve 

 between and beyond these is filled up with muscular impressions. The 

 muscle whose function was the closing of the shell has formed a small, 

 elongated, mesial, heart-shaped scar, and under, as well as along the 

 outer side are seen the impressions of the cardinal or divaricator muscles 

 — that is to say, of those which had the office of opening the shell. The 

 impressions of the pedicle or ventral adjuster muscle may also be clearly 

 or distinctly seen on either side close to the abductor ; so that the muscular 

 impressions appear to have been very similarly placed in this valve of 

 Athyris as in the corresponding one of Terebratula, although no trace 

 of the accessory divaricator and capsular, or peduncular, muscle could 

 be perceived ; but it is right to observe that these are not always dis- 

 tinctly seen even in the Terebratulae. 



In the interior of the smaller valve the hinge-plate presents four 

 depressions or pits, which afforded attachment to the dorsal pedicle or 

 dorsal adjustor muscles, and which served, according to Mr. Hancock, 

 to move the shell on the peduncle, and to adjust it. 



This hinge-plate is likewise perforated close to its summit (under the 

 umbone) by a minute circular aperture, which in some species has been 

 seen to communicate with a small cylindrical tube, which, after 

 originating under the platform, extends longitudinally and freely, with 

 a Blight upward curve, to about a third of the length of the valve. On 



* The term CUiothyria had been proposed some years before by Professor 

 Phillips for the same kind of shells, but its etymology is liable to the same ob- 

 jection as that of Athyris. On the Continent, the generality of authors refuse to 

 adopt either of these denominations, and prefer that of Spirigera, more recently 

 proposed by D'Orbigny, and I would willingly adopt this last denomination were 

 it not that, both in this particular and many others, naturalists still retain names 

 i^yhich imply zoological contradictions. 



