Morphology of the Blastoidea. 285 



do not understand what numerical difference he does take 

 into account in his classification of the Pentremites, unless it 

 be the difference in the number of pore-plates in the ambu- 

 lacra, a most unreliable character, as every Echinoderm 

 student is aware. If the bodies of all Pentremites have 

 the same number of plates arranged in the same relative 

 positions, what are the characters of the exoskeleton which 

 render it of such great importance in classification as Mr. 

 Hambach asserts ? 



I am also at a loss to know how differences in the " com- 

 position " of the plates can be of any classificatory value, as 

 stated in " the general rule which governs " Mr. Hambach's 

 classification of the fossil Echinoderms ; for I have always 

 imagined that these plates consist principally of carbonate of 

 lime, and are therefore of a tolerably uniform composition. 

 Perhaps Mr. Hambach will say that I am misrepresenting 

 him, and that he did not use the word " composition" in its 

 chemical sense. But it would be desirable to know what 

 meaning he does attach to it in the general rule already re- 

 ferred to. I do not imagine that he can have employed it 

 as denoting the relative shapes and sizes of the calycular 

 plates. For he tells us * that " it is arbitrary and without 

 any good reason to form of a certain number of species a new 

 genus (see Carpenter, loc. cit.) because their base plates are 

 small and depressed or elongated, or having a narrow, short 

 or long sinus in the fork pieces, which, if such is the case, 

 must necessarily give a different aspect to the individual, and 

 cause them to be respectively either globose, elliptical, pyri- 

 form, or clavate, which forms are met with in both those 

 having a broad or narrow ambulacral field. The number of 

 hydrospiric plicas can hardly be of any consequence, as shown 

 above." 



It may be well to state here that in speaking of the struc- 

 ture and distribution of the hydrospires as being of much 

 systematic value I was not alluding to the number of folds 

 which occur at the sides of the ambulacra. There is much 

 variation in this respect within the limits of genera, and some- 

 times even of species, e. g. Codaster acutus and C. trilobatus. 



Mr. Hambach omits to give any reference to the new genus 

 which I am said to have established for certain species upon 

 the characters which he mentions ; and I am therefore con- 

 strained to ask him either for its name or for the missing 

 reference to the page on which it was described. Unless he 



* Trans. St. Louis Acad. vol. iv. p. 5A7. 



