GENERA AND GROUPS OF THE ECHIJiTOIDEA. 175 



Few genera of the Echinoidea have received more attention 

 than Echinolrissus, Breynius, and Nucleolites, Lamarck. They 

 have beea most satisfactorily and candidly studied by Cotteau 

 and A. Agassiz during late years, and have been united by the 

 last-named author. He has shown that the singularly insufficient 

 character which was suj)posed to separate the genera, namely, 

 the presence or absence of grooving between the pores of a pair, 

 may be seen in the same petal of a specimen of a species. 



The so-called conjugation of pores of a pair is of no physio- 

 logical importance, and taken alone is of no classificatory value ; 

 but when it occurs with other characters it may assist to group 

 sets of species together in a genus. The obliquity of the 

 peristome is of greater importance, and, all other characters being 

 the same, it is of subgeneric value ; but if other characters of 

 importance differ, the obliquity should be of generic value. But 

 in EcTimohrissus the obliquity is of subgeneric value, and relates 

 to the forms which d'Orbigny, not very felicitously, called Tre- 

 matopygus. 



The possibility of retaining the recent species in the genus is 

 a matter of doubt, and as they have modifications of the pori- 

 ferous zones below the petaloid portions, they should come under 

 a subgenus Oligopodia. 



Grenus Echinobrissus, Breynius, 17o2, Schediasma de EcMn. 

 p. 62. Gray, 1825, Ann. PUl. p. 7 ; 1855, Cat. Ech. Brit. 

 Mus. p. 37. Milne-Edwards, 183(3, Cuvier's Beg. Anim. 

 ed. iii. Desor, 1858, Synopsis, pp. 257, 263. Zittel, 1864, 

 Novara Beise, Foss. Molusc. u. Ech. a. N. Zealand, p. 62. 

 A. Agassiz, 1872-74, Bevision of the Echini, pp. 555 & 557. 

 Cotteau, 1884, Bull. Sac. Zool. France, vol. ix. p. 336. Dun- 

 can, 1887, Quart. Journ. ^ol. Soc. xliii. pp. 420 & 429. 

 Bell, 1887, Ann. ^ Mag. Nat. Rist. ser. 5, vol. xx. p. 125. 



Syn. Nucleolites, Lamarck, 1801; Agassiz, 1847, Cat. rais., 

 Ann. des Sci. Nat. vii. p. 153. Trematopygus, d'Orbigny. 



Test rather thin, depressed, elongate, rounded in front, broadest 

 and more or less truncated behind ; or square, with the angles 

 rounded; or subcircular; tumid above, concave actinally. Abac- 

 tinally grooved posteriorly. 



Apical system subcentral or excentric in front ; four perfo- 

 rated basal plates ; madreporite in right anterior basal, or extend- 

 ing into others, or separating the lateral basals of one side from 



