121 



ture in the perisome at the ambulacral centre. It also becomes evident 



by the comparison that in general, the pakeozoic species resemble the 



embryonic stages of some of the recent Echinoderms, and that in these, 



(Bipinnaria for instance), the mouth is interradial. Rules such as are 



relied on in this case, afford a certain amount of presumptive evidence, 



■which, however, cannot prevail against material and visible facts. When 



we can see clearly that there is no aperture in that point, in the vault of 



a Crinoid, beneath which we know the ambulacral centre is situated, it is 



perfectly useless to supply one by theory.* 



The second objection is, that many of the fossils have a Plafyceras 



attached to them, in such a position as to cover the aperture which I call 



the mouth, and under such circumstances as to induce the belief that it 



lived parasitically on the Crinoid. The only answer I can make to this 

 is that, admitting the facts, we must suppose that space was left for a 



stream of water to pass under the edge of the shell, into the mouth of the 



Crinoid. In general, where one animal lives parasitically upon another, it 



does not destroy its host. Some of the gasteropods of the Devonian and 



Carboniferous ages were carnivorous, as is proved by the bored shells and 



Crinoids that are occasionally found. I have seen a number of such 



specimens, and several years ago I read apaper on the subject (which was 



never published) before the Natural History Society of Montreal. There 



were several good Conchologists present, and the specimens exhibited were 



compared with bored shells of existing species. All pronounced the style 



of workmanship to be precisely the same. I have the proboscis of an 



Aciinocrinns that is bored near the base, and among the fossils lent me 



Q by Mr. Wachsmuth, is a Codonitcs 



steUiformis, that is bored through one of 



the ambulacra. The view I took of the 



subject in my paper, was that the gastero- 



pod ascended the stalk of the Crinoid and 



thrust its proboscis into the mouth of the 



Fig. 82 sireptorhynchus Pan- latter. The Crinoid then slowly drew 



dora. A specimen bored at o by .^^ ^^^^ together, and held thc shell fast 

 a carnivorous gasteropod. From _ 

 the Corniferous Limestone, Devo- until both died, 

 nian, Canada. 



* The position of the ambulacral centre may thus be found. When the mouth is 

 eccentric, the ambulacral tubes usually converge to the centre of the vault. But when 

 the mouth is central, we first find the azygos interradius, in general easily recognized by its 

 possessing a greater number of plates than do any one of the other four interradii. On the 

 opposite side of the fossil is the azygos arm. The ambulacral centre is always situated 

 betwecn;.this aim and the mouth, never on the side of the mouth toward the azygos inter- 

 radius. 



