302 L. Agassiz on the Echinodermata. 



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ambulacrum scarcely perceptible ; the four ambulacra] pairs de- 

 pressed, forming at the summit of the disc a kind of cross, circum- 

 scribed by a sinuous line without tubercles or spines. I am not ac- 

 quainted with any fossil S] 



B. pectoraUs, Ag. (Spatangua Pect., Lam.) — B. carinatus, Leake. — B. 

 columbarit, Lam. — B. ScilUe, Ag. (Echinus Spatangua Scilla.) — B. unicolor, 

 Leake. — B. oentricostu, Leske. — B. compressus, Ag. (Spat, compr., Lam.) — 

 B. stenialis, Ag. (Spat, stern., Lam.) 



9. Schizasteb, Ag. (Echinocardium,\ .Ph. and Gr. — Spatangus, 

 de Bl. Sect. 13.) — Disc heart-shaped, very much raised posteriorly ; 

 anterior dorsal groove long and deep ; four other grooves at the dor- 

 sal summit deep and narrow, in which the ambulacra are hidden. 

 One fossil species, and one living. 



Scli. Atropos, Ag. (Spat. Atr., Lam.) — Sch, Studeri, Ag. (from the Italian 

 tertiary.) 



II. The Clypeastres occupy the intermediate place between the 

 Spatangi and the Cidarites ; their form is most generally circular. 

 The mouth is central or subcentral ; but the anus is more or less 

 approximated to the periphery, and is found sometimes at the upper 

 surface, sometimes at the under surface of the disc. 



1. Catopygus, Ag. (Nucleolites, auct.) — Disc oval; ambulacra 

 converging uniformly towards the summit; anus at the posterior 

 surface. All the species are fossil, from the jura, chalk, and ter- 

 tiary deposits. 



C. semi-globosus, Ag. (Nucleolites semi-gl., Munst.) — C. carinatus, Ag. 

 (Nucl. carin., Goldf.) — C. castanea, Ag. (Nucl. cast., Al. Br.) — C. pirifor- 

 mis, Ag. (Nucl. pyrif., Goldf.) — C. ovulum, Ag. (Nucl. ov., Lam.) — C. de- 

 presses, Ag. (Nucl. depr., Al. Br.) — C. subcarinatus, Ag. (Nucl. subcar., 



Goldf.) — C. obovatus, Ag. 



2. Pygaster, Ag. (Nucleolites and Clypeus, auct.) — Disc circu- 

 lar ; ambulacra converging uniformly towards the summit ; orifice 

 of the anus large at the upper surface of the disc. Species all fos- 

 sil, from the jura and chalk. 



P. semisulcatus, Ag. (Clyp. semisul., Phil.) — P. depresses, Ag. (Nucl. 

 depr., Munst.) 



3. Galerites, Lam. (Conulus, Klein. — Echinochonus, De Bl.) — 

 Disc circular ; ambulacra narrow, pierced with pores rather distant 

 from one another, converging uniformly towards the summit ; mouth 

 central, anus marginal and inferior. Species all fossil, from the chalk. 

 This genus approaches more to the Nucleolites and Echinonece than 

 to the true Echini. 



