192 THE CANADIAN NATURALIST. [June 



known existing species, the mouth is in the centre, but it does 

 not certainly follow that it is so in all the Echinodermata, living 

 and extinct. Whether it be so or not in any particular fossil 

 species, whose structre may be under investigation, is a question 

 of wliich fact can only he j^ositively determined hy direct observa- 

 tion of specimens. On appealing to these we find that, in a large 

 proportion of the fossil forms, there is no aperture in the peri- 

 some at the ambulacral centre. It also becomes evident by the 

 comparison that, in general, the paleozoic species resemble the 

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

 these, (^Bipinnaria for instance), the mouth is interradial. Kules 

 such as are relied on in this case, afford a certain amount of pre- 

 sumptive evidenc3, which, however, cannot prevail against mate- 

 rial 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 deduction. ^^ 



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

 ceras attached to them, in such a position so as to cover the aper- 

 ture 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 his host. Some of the gasteropods of the Devonian 

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

 bored shells and Crinoids that are occasionly found. I have seen 

 a great number of such specimens, and several years ago I read a 

 paper on the subject (which was never publiished) 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 



* The positiou of the ambulacral centre may thus he 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 tho other four iuterradii. 

 On the opposite side of the fossil is the azygos arm. The ambulacral 

 centre is always situated between this arm and the month, never on the 

 side of the mouth torward the azj^gos interradius. 



