78 SCHIZ ASTER 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 S. canali/ens, >S. fragilis, and S. Philippii, 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 has an extensive geographical 
range, having been collected by the Challenger in the Arafiira 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 study of the young of several species of 
Schizaster in the earliest stages of growth, they have a globular test, the 
ambulacral 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'. orhignjimius, S. fragilis, and S. japon- 
icus. 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. 
Moselegi. 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 Schizasterida?, 
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, and 
the madreporic body is elongate, extending longitudinally over the greater 
part of the apical system. 
A^ V ct/UxrV^ i ^\oi 
