PEOCEEDINGS OF GEOLOGICAL SOCIETIES. 



489 



The author commenced by indicating the position of the Echinidse as a large 

 and important section of the Radiata, and pointed out the several steps in the 

 series which connected the completely circular types with those havmg a marked 

 tendency towards bilaterality. 



The principal characteristics of the order were then enumerated, commenc- 

 ing with a description of the calcareous shell, or test, constituting an internal 

 and integral portion of the animal being secreted by and enclosed within 

 organized membranes, and participating in the life of the organism — the homo- 

 logiie of the vertebrate skeleton rather than of the shell of the moUusca. 



The several elements of the tests were then described, the ten columns of 

 small plates constituting the ambulacra, the ten larger, the iuterambulacra, 

 separated from each other by ten rows of holes — the poriferous zones. The 

 tubercles with which these plates are studded were then enumerated — the large 

 primaries, the secondaries, the miimte granules, and the miliary granulation. 

 The varities of form and arrangement of the pores were next reviewed, and 

 then the relative position and varying shapes of the two great orifices, the 

 receptive andrejective poles. The internal organization was briefly glanced at, 

 and the principal viscera enumerated. The apical disc was described, its 

 separate elements, the ovarian, and ocular plates, theii' relative nature and posi- 

 tion indicated. 



The varieties of spines, their forms, sculpture, attachment, and the structure 

 of their parts, was also pointed out. « 



The author next proceeded to dwell upon the relative value of the external 

 organs in classification, begimiing with the position of the mouth and vent, and 

 especially urging attention to the physiological import of these and the other 

 organs, and even of scidptm^al decoralion of the test as indicative of purpose 

 and design, and not to be considered merely valuable as means of classiiica- 

 tion. He dwelt upon the necessity of taking into consideration the mutual 

 relations of the great vital functions of digestion, reproduction, vision, and 

 locomotion as the basis of any sound natural arrangement. The position of 

 the vent either within or external to the elements comprismg the apical disc 

 was shown to be a good character by which to divide the order into two great 

 divisions — the Exocyclic and the Endocyclic Ecliinidae ; the structure of the 

 ambulacra and poriferous zones, taken in connection with the position of the 

 vent, formed good secondary characters for grouping the genera into natural 

 families, while the former were well defined by the form, number, and arrange- 

 ment of the spines and tubercles, the miliary granulation, the size and number 

 of elements of the apical disc, and the position of the vent taken collectively. 

 The author then intimated that the limits he had assigned to himself did not 

 permit of any indication of specific characteristics, but that he should content 

 himself with an enumeration of the principal poiuts of the generic classification 

 of the Echinidse of the chalk, and of the families to which they belonged. 



He then proceeded to describe by the aid of numerous large diagrams the 

 generic distinctions of Cidaris, Diadema, Cyphosoma, Echinus, Salenia, Dis- 

 coidea, Galerites, Cacatomus, Nucleolites, Catapygus, Pyrina, Holaster, 

 Cardiaster, Ananchytes, Micraster, Hemipneustes, Hemiaster. 



A vote of thanks having been passed, the author stated that in selecting this 

 subject he had been guided by an old admiration of an order which, from 

 living in a chalk-district, had attracted his attention in early youth, and he 

 had often felt the want of such a guide as the observations he had just made 

 would furnish. He particularly denied all claim to originality, his only merit 

 being that of having collected into one view, materials which lay scattered in 

 a number of rare and costly works. 



