300 L. Agassiz on the Echinodermatd. 



I. The Spatangi have the body more or less elongated and gibbous ; 

 their mouth is furnished with jaws, and is placed towards the anterior 

 extremity, and the anus towards the posterior extremity, sometimes 

 on the upper surface of the disc, sometimes on the lower. Their shell 

 is thin, covered with small tubercles, very- numerous, among which 

 are observed some larger ones, which are scattered and often per- 

 forated like those of the Cidaritcs. The spines are setaceous, often 

 compressed, and of unequal size. The anterior ambulacrum is gene- 

 rally less developed than the rest ; they form round the mouth 

 grooves, where the holes are larger and whence proceed ramified 

 tentacula like those of the Holothurise. There are only four ovidu- 

 cal plates, which are very distinct. 



1. Disaster, Ag. (Spatangus , Ananchytes , etNudeolites, auct.) The 

 odd ambulacrum and those of the anterior pair converge at a point 

 situated at a greater or less distance from the point of junction of 

 the two posterior ambulacra. All the species of this genus are fossils 

 of the chalk or of the jura. 



D. bicordatus, Ag. (Spatangus bic, Gold/.) — D. ellipticus, Ag. (Anan- 

 chytes ellipt, Lam.) — D. excentricus, Ag. (Nucleolites excentr., Munst.) — 

 D. canaliculars, Ag. (Nucleo. canal., Munst.) — D. granulosus, Ag. (Nu- 

 cleo. granul. Munst.) — D. capistratus, Ag. (Spat, capistr., Gold/.) — D. cari- 

 natus, Ag. (Spat, carin., Goldf.) — D. ovalis, Ag. (Spat. oval.. Park.) — D. 

 analis, Ag. — D. ringens, Ag. (these two latter from the Swiss Jura.) 



2. Holaster, Ag. (Spatangus, auct.). — Disc heart-shaped ; ambu- 

 lacra converging uniformly towards a point at the summit ; anus 

 superior. All fossils, especially of the chalk. 



H. granulosus, Ag. (Spat, gran., Goldf.)-— H. hemisphcericus, Ag. (Spat, 

 hemisph., Phil.) — H. Icevis, Ag. (Spat, lsev., Deluc.) — H. nodulosus, Ag. 

 (Spat, nod., Dolf.) — H. planus, Ag. (Spat, plan., Mant.) — H. complanatus, 

 Ag. (Spat, compl., De Bl.) — H. intermedins, Ag. (Spat, interm., Munst.) — 

 H. subglobosus, Ag. (Spat, subgl., Leslie.) — //. sub orbicular is, Ag. (Spat, 

 suborb., Defr.)—H. truncatus, Ag. (Spat, trunc., Goldf.) 



3. Ananchytes, Lam. andDe Bl. (Echinocorys, Breyn. and Gray ; 

 Galea and Galeola, Klein.) — Disc oval, no groove along the anterior 

 ambulacrum ; anus oblong, placed longitudinally ; ambulacra con- 

 verging uniformly towards the summit, where the double pores are 

 very close, while they are widely apart at the circumference. All 

 the species are fossils from the chalk ; they have been too much 

 multiplied from mere differences of age. 



A. ovata, Lam. — A. gibba, Lam. — A. hemisphcBrica, Al. Br. — A. pustu- 

 losa, Lam. is but the inner cast of A. ovata. — A. quadriradiata, Leske, is 

 merely a monstrosity. 



4. Hemitxeustes, Ag. (Spatangus, auct.) — Disc heart-shaped, 



