ECIIINOBRISSUS. 331 



The inter-ambulacral areas are wide ; the plates composing these portions of the test 

 are large, and bent in the middle ; each plate supports about three rows of small tubercles, 

 which are arranged in lines, with much regularity (PI. XXXII, fig. 1 c). 



The EcHiNOBRissiD/E have many organic characters in common with the Echino- 

 LAMPiDyE, but they are distinguished from that family by the position of the vent, and by 

 the absence of the poriferous petals which in the latter surround the mouth. 



The EcHiNOBRissiDiE, thus limited, nearly corresponds with the group of Nucleohdees 

 in the family Cassidulees of Agassiz and Desor; it differs from the family NucleolitiDjE 

 of Dr. Albin Gras, and the family EchingbrissiDjE of M. d'Orbigny, by the elimination 

 of all the urchins therefrom which have a basal vent, peristomal lobes and petals, and an 

 organization in accordance with the genus Echinolampas. 



6'e;?«5— ECHINOBRISSUS, Breynius, 1732. 



NucLEOLiTES, Lamarck, 1801. 

 ~ Goldfuss, 1826, 



— Agassiz, 1837. 

 EcHiNOBRissus, B'Orbigny, 1855. 



— Desor, 1857. 



— Cotteau, 1858. 



This natural group is composed of small urchins, which have an oval, oblong, sub- 

 quadrate, or sub-circidar form, more or less convex on the upper surface, and slightly 

 concave beneath ; the test is rounded anteriorly, more or less produced, truncated, or 

 lobed posteriorly, and is in general narrow before, and wider behind : the vent opens in 

 the upper surface into a dorsal valley, which in one section extends from the apical disc to 

 the posterior border, and in another is limited to the lower half thereof; the aperture is 

 closed by a series of small anal plates, which are usually absent in fossil species, but are 

 preserved in the only living example of the genus at present known (PI. XLI, fig. 1). 



The base is more or less concave ; the small mouth-opening is excentral, and 

 lodged in a depression, nearer the anterior than the posterior border; the peristome in 

 many of the species is regularly pentagonal, in others it is oblique. M. d'Orbigny* has 

 separated the latter into a distinct genus, under the name Trematopygus, but these oblique- 

 mouthed species can at most be considered only as a section of the genus Echinohrissus ; 

 which are supposed to be special to the Cretaceous Rocks. 



The apical disc is small, square, and compact ; it is composed of four perforated 

 genital plates, arranged in pairs, and one single imperforate plate; the right antero- 

 lateral is the largest, and supports the madreporiform tubercle ; the five small 



* ' Pal^ontologie rran9aise Ter. Cretaces,' tome vi, p. 37-1. 



