GENEBA AND GEOITPS OF THE ECHHiTOIDEA. 233 



M. Cotteau, Ech. Foss. de I'Algerie, 1885, fasc. 9, p. 68, men- 

 tions that M. Pomel considers that P. Nicaisei, Pom., has an 

 apical system like Micraster, but that he finds three genital pores. 

 He states that the greater number of species have only two genital 

 pores. This is but one of the many proofs that the number 

 of genital pores in a species is not a guide to its generic position. 



Agassiz regarded Pericos«?ws as his third diWi^ion o? Semiaster. 



G-enus Likthia, Ifej^ian, 1853, JV^ote sur les Ech., Actes Soc. 

 Helv. p. 278. Besor, 1853, Synopsis, p. 395. A. Agassiz, 

 1872-74, Bevision, pp. 138, 604 ; 1881, Beport on ' Chal- 

 lenger ' Ech. pp. 199, 204. Loven, 1874, Etudes, pi. xxviii. 

 E. A. Smith, 1878, Ann. Mag. Nat. Hist. ser. 5, vol. i. p. 67. 

 Cotteau, 1887, Bull. Soc, Zool. France, vol. xii. p. 556. 

 Syn. Desoria, Gray, 1851 ; Periaster, d'Orb., 1854. 

 Test variable, small to large, oval or cordiform, grooved ante- 

 riorly, subacuminate or truncated posteriorly, tumid and gibbose 

 dorsally, almost flat actinally. 



Apical system small, excentric in front ; four perforated basal 

 plates ; the madreporite separating the posterior basal plates and 

 also the posterior radial plates. 



Ambulacra diverse ; the anterior in the broad groove, the 

 pores round and small ; the antero-lateral long, with the petaloid 

 parts in grooves, moderately long, divergent, pairs of pores 

 equal or subequal, nearly closing distally ; postero-lateral ambu- 

 lacral petals also in sunken grooves, less divergent and shorter 

 than the others. Ambulacra forming the greater part of the 

 peristomial margins, and moderately broad on either side of the 

 sternum. 



Peristome exceatric in front, semilunar, with a well-developed 

 posterior labrum. An amphisternum ; the second plates of both 

 of the zones of the right posterior ambulacrum united, so as to 

 produce ancient heterouomy. Periproct at the upper part of the 

 posterior truncation. 



A peripetalous fasciole entering the interradia, a lateral 

 fasciole starting from the peripetalous close to the antero-lateral 

 ambulacra and passing beneath the periproct. Tubercles crowded, 

 largest actinally, usually crenulate and perforate, and either on 

 flat or in sunken scrobicules. 



Fossil. Cretaceous : Europe, Africa, JST. America. Tertiary : 

 Europe, Africa, W. Sind, W. Indies. 



