444 H. L. Hawkins — Morphology of Echinoidea. 



Pyrina). It is perhaps reasonable to complain of the laxity of 

 Agassiz, Desor, andde Loriol in not allowing sufficient weight to the 

 characters of the peristome, but surely the apical system deserves a 

 little notice. If there is one constant feature that differentiates the 

 Pyrina- Conulus-Echinoneus series from the other Holectypoida, it 

 is the absence of a posterior genital plate. All three genera have 

 almost identical apical systems, and to admit a species that normally 

 differs in so important a character as the presence of an additional 

 plate is to subvert taxonomy. However, there is at least a possibility 

 that the "small, imperforate, fifth genital" of P. houzeaui may 

 be an individual variant, even if it has any real existence. Such 

 supplementary plates do occur occasionally in the apex of Conulns, 

 presumably as expressions of regressive variation, and cases have not 

 been wanting when a fifth genital has been ascribed to that genus as 

 a normal feature. Lambert (I.e.) corrects the figure of the apical 

 system of Globator nucleus given by Desor, stating that the fifth, 

 imperforate genital does not exist. However (p. 149), he refers to a 

 "rudiment of the fifth genital" as occurring between the posterior 

 oculars in Pyrina ataeina (atacica, Cotteau), but it is not clear whether 

 his description is based on one specimen or is a generalization. If 

 this is a normal feature in the species, it constitutes not merely 

 a morphogenetic point of the utmost interest, but, to my mind, 

 a reasonably sound basis for generic separation. However, at present 

 I incline to regard the occurrence of posterior genitals in these forms 

 as individual variants, interesting and instructive, but abnormal. 



The foregoing paragraphs are intended to explain the meaning here 

 attached to the three generic terms used as the title of the paper. 

 That further subdivision of Pyrina and Gonulus will prove necessary 

 in the future I not only expect, but believe; but for the present they 

 serve as well-marked, though closely similar, genera. It is important 

 to lay stress upon the striking resemblances that all three types show 

 to one another, since it is usual and orthodox to admit Conulus into 

 the Holectypoida, and to relegate Pyrina and Echinon'eus to the 

 Spatangoida. One purpose of the present paper is to contend thatthe 

 three genera are too closely related to be separated by even family 

 distinction, and that the ruling of an ordinal line through the group 

 is unnatural. 



The folloAving are the chief morphological resemblances : — 



Shape. — Antero-posterior diameter always greater than transverse. 

 The degree of elongation of the test varies much within the genera, 

 but the two diameters never become quite equal, save perhaps in very 

 young examples. Height extremely variable, usually less than the 

 transverse diameter except in late species of Conulus. 



Peristome. — Always to some degree oblique, elongated approximately 

 on the axis II, 4. The obliquity is most marked in Echinon'eus, where, 

 however, the peristome of the young is almost perfectly symmetrical 

 (decagonal). 



Perignathic girdle. — Discontinuous, strongly inclined, and well 

 buttressed in Conulus, apparently similar in Pyrina, but wholly 

 wanting in adult Echinon'eus, where the vestigial girdle of the young 

 is very imperfectly developed. 



