476 MR. HERBERT L. HAWKIIS^S ON 



I have been unable to find any record of the preservation of 

 radioles in Discoidea. In Conulus the primaries are very much 

 like those of the adoral surface of Holectypus in shape and 

 ornament. The miliaries support curiously blunt prominences, 

 which are usually preserved in situ, but are very easily rubbed off 

 by too vigorous development of the specimen. These blunt spines 

 were figured by Forbes (14), who also gave a drawing of a cui-ious 

 body that he regarded as a pedicellaria. The characters of the 

 miliary spines suggest a comparison with the calcareous supports 

 of pedicellarije ; but if they all had this function, the number of 

 those organs would be extraordinarily great in proportion to the 

 radioles. 



Nothing seems to be known as yet of the microstructure of the 

 radioles of any genera of the group, as Hesse (51) did not choose 

 an Holectypoid for his researches. 



The slight progressive change traceable in the primary radioles 

 seems to lead merely to an increase in the length, and pei-haps in 

 the slenderness, of their shafts ; while, as would be expected from 

 the diflference in size of the tubercles on the two surfaces of the 

 test, the adoral radioles become proportionately longer than those 

 of the adapical surface. 



G. Internal Buttresses. 



The difference in form which makes so violent a contrast between 

 a Scutella and an Echinus must result in a corresponding difference 

 of resisting power against the pressure of the waves. As both 

 types of Echinoid may live between tide-marks, where the violence 

 of the waves is most felt, they both have the same forces to repeh 

 A spherical body such as that of an Echimis, or even a hemi- 

 spherical one, like that of an Holectypus, could easily ward ofi" the 

 blow of a breaking wave, in the same manner as a Patella does. But 

 a flat test, such as that of a ScutelHd, would offer a blank resistance 

 to the waves, and, if hollow, would almost certainly be crushed. 

 For this reason, the few groups of the Irregular Echinoids that 

 frequent the exposed littoral habitat so characteristically occupied 

 by the Regular forms, strengthen the resisting power of their 

 tests by the development of massive calcai'eous buttresses within. 

 Practically the only Irregular forms which live openly on the shore 

 at the present day are the Clypeastroida. It becomes, therefore, 

 a point of especial interest to find the beginnings of internal 

 supports to the test among the Holectypoida, which is the only 

 other gnathostomatous (and therefore rock-dwelling) order. 

 Although the development of the buttresses in the two groups 

 might easily be regarded as an illustration of similar adaptation 

 to similar environment alone, yet, in the light of the other less 

 obviously utilitarian features of the two groups, it seems in this 

 case that a genetic explanation exists as well. 



On the internal mould of a Pygaster, and yet more in one of a 

 Pileus, besides the deep pits left by the prominences of the 



