the Crinoidea, Cystidea, and Blastoidea. 153 



we can see clearly that there is no aj)erture 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 de- 

 duction*. 



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

 Platyceras attached to them in such a position as to cover the 

 apertm-e 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 Gas- 

 teropods of the Devonian and Carboniferous ages were carni- 

 vorous, 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 a paper 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 Actino- 

 crinus that is bored near the base ; pjo-. 12. 



and among the fossils lent me by 



Mr. Wachsmuth is a Codonites 

 stelliformisthat has one of the am- 

 bulacra bored through. The view • 

 I took of the subject in my paper 

 was, that theGasteropod ascended 

 the stalk of the Crinoid and thrust 

 its proboscis into the mouth of 



the latter. The Crinoid- then specimen bored at o liy a carnivorous gas- 



1 1 1 •. J. il 1 teropod. From the Corniferous Lime- 



Slowly drew its arms together, and gtone. Devonian, Canada. 



held the shell fast until both died. 



A third objection is the small size of the aperture in some 

 of the species. In general, where there is no proboscis, the 

 orifice is from one twentieth to one tenth of an inch in dia- 

 meter, quite sufficient for an animal that subsists on micro- 



* The position of the ambulacral centre may thus be found. Wlien 

 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 gi-eater 

 number of plates than any one of the 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 mouth, never on the side of 

 the mouth towards the azygos interradius. 



Ann. & Mag. N. Hist. Ser. 4. Vol vii. 1 1 



Fig. 12. Sfreplorh t/nckus pandora. 



