ECHINODERMATA. 21 



sometimes it is transversely oblong. Anal opening transversely oblong, and infra-marginal ; 

 apical disc small, excentral, with four genital holes. Some species are living in warm 

 seas, but the greatest number are extinct, and distributed in the Oolitic, Cretaceous, 

 and Tertiary rocks. 



Types. JEchinolampas orientalis. Gray. Pygurus depressus, Agassiz. Conoclypus 

 Leshii, Goldfuss. 



Family 11. Clypeasterid^. — Test thick, elevated or depressed, circular, elliptical, or 

 pentagonal ; the surface closely covered with small, nearly equal-sized tubercles, sunk in 

 the plates, and surrounded by ring-like areolas, the tubercles carrying short hair-like spines. 

 The mouth large, central and pentagonal, and armed with five strong jaws, which carry the 

 same number of teeth. The anus posterior, marginal or infra-marginal ; the interior of 

 the test is sometimes divided by pillar-like processes formed of the inner layer of the plates. 

 The dorsal portions of the ambulacral areas have a petaloid form, circumscribed by large 

 poriferous zones ; the basal portions are narrow, rectilineal, or branched ; the five genital 

 plates form a circle around the madreporiform body, and between these are wedged the 

 five ocular plates. This family includes the genera Clypeaster, Lamk., Laganmn, Klein., 

 EcMnarachnius, Van Phels., Arachnoides, Klein., Scutella, Lamk., Dendraster, Agass., 

 Lobophora, Agass., Encope, Agass., Echinodisciis, Breynius, Mellita, Klein., Buna, Agass., 

 Moulinsia, Agass., Scutellina, Agass., EcJdnocyamus, Van Phels., Fibularia, Lamk., 

 Lenita, Desor. 



Types. Clypeaster rosaceus, Lamarck. Scutella subrotunda, Lamarck. EcJdnaracJmius 

 placenta^ Gmelin. 



Family 12. Echinocorid^. — Test thick, oval and elevated, sometimes cordate; 

 ambulacral areas narrow ; poriferous zones narrow, pores unigeminal ; test covered with 

 small tubercles, which are perforated and crenulated. One genus has a marginal fasciole. 

 Mouth opening small, excentral, transversely oblong. Anal opening nearly of the same 

 size, oblong, marginal, or supra-marginal. Apical disc elongated, nearly central, with four 

 genital pores ; the cordate forms have an anterior central depression. The species are 

 all extinct, and are limited to the Cretaceous rocks. 



Types. EcMnocorys vulgaris, Breynius. Holaster sub-globosus, Leske. Cardiaster 

 granulosus, Goldfuss. 



Family 13. SpATANoiDiE. — Test thin, oval, oblong or cordiform, and satisfactorily ex- 

 hibiting the bilateral symmetry of the Echinoidea, The anal opening is posterior and supra- 

 marginal, and is closed by a complicated series of small plates. The apices of the ambu- 

 lacral areas are united at the summit of the test. The anterior single ambulacrum has a 

 different structure from the antero- and postero-lateral pairs, and is in general lodged in a 

 depression of the test, which extends to the anterior border, forming the anteal sulcus ; the 



