78 SCHIZASTER LIMICOLA. 



Hemiaster, and the like. The structure of the odd anterior ambulacrum of 

 this species has all the characteristics of the odd ambulacrum of such species 

 of Schizaster proper as 8. camUferus, S.fragilis, and S. PkiMppu, while it has 

 the outline of the test of Hemiaster and the fascioles of Periaster. There are 

 but two genital openings. When brought up in the dredge, this species was 

 of a dirty yellowish-brown color. This species lias an extensive geographical 

 range, having been collected by the Challenger in the Arafura Sea. 



The Schizasterids which have been united under the generic name of Peri- 

 aster are interesting as representing a truly embryonic stage of Schizaster 

 proper. As is well known from the stud}' of the young of several species of 

 Schizaster in the earliest stages of growth, they have a globular test, the 

 ambulacra! grooves are either indistinct, or only slightly developed, or totally 

 absent. The peripetalous fasciole is broad, but slightly indented ; in fact, all 

 the characteristic features of the genus are those which we find in the young 

 stages of such species of Schizaster as S. orUgnyanus, S.fragilis, and S.japon- 

 tens. If we were to magnify a young S.fragilis measuring about 10 mm. in 

 diameter about six times, we should have a fair representative of the genus 

 Periaster, provided we took into account the great variation of the lateral 

 fasciole, which is more or less indistinct as it passes towards and under the 

 anal system to form the anal fasciole in such species as S. fragilis and S. 

 Mosclei/i. The odd anterior ambulacrum is, like all the other ambulacra, but 

 slightly sunken, and indicated by a broad re-entering angle at the ambitus, 

 neither of the lateral pairs of ambulacra being as deeply sunken as in 

 Schizaster proper, the posterior pair being, as in most of the Schizasteridae, 

 shorter than the anterior pair. The relative proportions of the lateral 

 ambulacra recall to a certain extent the structure of the ambulacra of the 

 older Spatangoids of the Chalk, such as Hemiaster proper. In large speci- 

 mens of Periaster limicola measuring 68 mm. in length, there are two pos- 

 terior genital openings, as in Hemiaster, the ocular plates are large, ami 

 the madreporic body is elongate, extending longitudinally over the greater 

 pint of the apical system. 



