Herbert L. Hawkins — Studies on the Echinoidea, etc. 201 



depressed structures known as " glassy tubercles". Their structure 

 and distribution are admirably figured by Westergren (Mem. Mus. 

 Comp. Zool., Harvard, 1911). These glassy tubercles are roughly 

 similar to the secondaries in size, but are distinguished from them 

 by their deeply sunken areolae and small "mamelons". Whatever 

 the glassy tubercles may be, either in origin or function, their 

 general facies is so extraordinarily like that of the " sunken tubercles " 

 of Discoides that it is almost impossible to doubt the homology of the 

 two structures. I have not noticed any specially vitreous appear- 

 ance in the mamelons of the "sunken tubercles" of Discoides, but 

 this might be due to fossilizationor the imperfect development of the - 

 structures in Cretaceous times. If, as I believe, corresponding 

 structures occur in Pyrina, which is very nearly allied to Echinoneus 

 in all essential features, and almost certainly ancestral to it, the 

 correlation of the two sets of ornament would be rendered more 

 certain. Their absence from Conulus would indicate that that genus, 

 though resembling Pyrina in many features, has diverged along 

 a line of evolution leading to some other goal than Echinoneus, and 

 one in which "glassy" or "sunken" tubercles are not developed. 

 For the present it is enough to definitely state the opinion that the 

 "sunken tubercles" of most Holectypoids are the forerunners of 

 the glassy tubercles of Echinoneus. 



3. The Spiniferous Pits of Conulvs albogalerus, Leske. 

 (a) Description of the Pits. 



This familiar and common species from the higher zones of the 

 Upper Chalk may present very different surface features under 

 varying conditions of preservation and preparation. Ordinarily the 

 surface of the test is almost smooth on the adapical surface, owing to 

 the faint relief and shallow areolae of the primary tubercles, and the 

 small size of the secondaries and miliaries. But most specimens 

 that have been freed from their adherent matrix by gentle methods 

 show, in contrast, a strongly shagreened character, which is caused 

 by the projection of very numerous rounded prominences of con- 

 siderable elevation but small diameter. These prominences are 

 readily detached from the plates by too vigorous brushing, and seem 

 to separate from them after a very slight degree of weathering; and 

 they as often as not are removed with the matrix if this is broken off 

 by a blow. That they commonly remain in position on tests 

 which have been entirely denuded of radioles and pedicellarise 

 indicates that these prominences are not appendages of that type, but 

 their frequent and easy separation from the plates with which they 

 articulate proves that they are not tubercles. They seem to occur 

 in almost the same numbers in C. rhotomagensis and C. subrotundus 

 as in C. albogalerus, but owing to the toughness of the matrix in 

 which the two former species are embedded, they are only 

 exceptionally seen in situ. 



The earliest description of the prominences scattered over the test 

 of C. albogalerus is that given by Forbes (Mem. Geol. Surv., dec. in, 

 pi. viii) in 1850, and no later accounts of them seem to add much to 

 his statements and figures. The following are the sections of his 



