the Skeletons of Motlusca, Crustacea and Echinodermata. 389 



be found among other species of Pentacrinus, and that indubi- 

 table specific characters (which are much wanted in this genus) 

 might be estabhshed upon these. 



In conclusion, I desire to make it known, among those who 

 may feel an interest in this inquiry and who may perceive its 

 importance, that I shall feel particularly obliged by being sup- 

 plied with specimens for examination ; that by extending the 

 basis of my inquiries, my general conclusions may acquire a va- 

 lue and stability, which I am conscious that they do not at pre- 

 sent possess. Single or imperfect valves of any of the rarer re- 

 cent genera, and fragments of almost any fossils in which the 

 shelly structure is well preserved, will be gratefully received by 

 me ; and I shall do my best to make them available for the pur- 

 poses of science. 



Addendum. 



Having been encouraged by the British Association to carry 

 out these investigations, and having been requested to furnish a 

 detailed Report on the Microscopic Structure of Shells, &c. for 

 its next meeting, I am particularly anxious to lose no time in ex- 

 amining as large a number of species, both recent and fossil, as I 

 may be enabled to do, by the kind assistance of those who may 

 be disposed to promote my object. Any contributions of the 

 kind specified above may be forwarded to me direct through the 

 post-office if not too bulky ; and Mr. Lovell Reeve, King William 

 Street, Strand, and Mr. Woodward, Geological Society, have kindly 

 undertaken to assist me, by receiving and forwarding to me any 

 specimens that may be transmitted to them. 



I may subjoin, as interesting results of observations made 

 since the meeting of the Association, the identification of the re- 

 markable structure of Pleurorhynchus Hibernicus (Cardium Hiber- 

 nicum) with that of Hippurites and other Rudistes ; leaving no 

 doubt in my own mind of their close affinity, this structure being, 

 so far as I know at present, peculiar to the last-named group. 

 It has given me great pleasure to learn, that several intelligent 

 conchologists had formed the same opinion on independent 

 grounds; though, so far as I am aware, it has never been put 

 forward in print, and I was entirely ignorant of it at the time 

 my own observations were made. The structure in question is 

 of a cancellated nature, filling up the space between the internal 

 and external layers of shell, in the same manner as the diploe 

 fills up the space between the two tables of the skull. The can- 

 celli are very regular in form and arrangement; and though 

 they are now filled with carbonate of lime, I think its appearance 

 indicates that it is the result of fossilization. I am not acquainted 

 with cancellated structure exactly resembling this in any recent 

 shell ; the nearest approach to it is presented by Balanus, Coro- 



