the Morphohgij of the Blastoidea. 245 



where the anus perforates one of the oral plates, while the 

 spiracles are partly formed by notches in its side*. The 

 spiracles vary but little in size, and, as a rule, are very small, 

 as in S. i^isuvij S. neglectuSj S. melo, &c., but of lai'ger size 

 in S. glaher. The anal spiracle is always very dispropor- 

 tionate in size as compared with the others. It is frequently 

 bounded on its outer margin by a tubercle or boss of greater 

 or less elevation, as in H. neglectus^ S. granulosus , and S.melo. 

 The anal opening is said to be circular in S. pisum, ovate in 

 S. Sagi'y and pyriform in S. vielo. The summit of S. meJo 

 presents some departures from the type described above, which 

 approximate it to Pentremites proper. This is especially marked 

 in specimens wdiich have the side plates in situ at the central 

 ends of the ambulacra. A wide median ridge rising from the 

 apex of each oral plate separates two spiracular openings, 

 which are apparently constructed upon the same type as those 

 of PentremiteSy viz. floored by oral plates and leading into a 

 hydrospiral canal, roofed in by side plates. 



The real state of the case, however, is as follows : — The 

 lancet-plate is separated from the radial plates by the thick 

 upper edge of the inner wall of the hydrospire-tubes. J\l inute 

 cross pieces between this plate and the sides of the radials 

 convert the hydrospire-cleft into a canal opening externally 

 by pores between these cross pieces, whilst the side plates, 

 resting on the latter, alternate with the pores as usual But 

 this hydrospire-plate terminates at the oro-radial suture ; and 

 at the central end of the ambulacra the lancet-plate comes into 

 direct and continuous contact with the orals without leaving 

 any intervening pores, although the side plates rest on it as 

 usual. Between the converging edges of the lancet-plate and 

 the large notches in those of the oral plate at its sides are the 

 spiracular openings, which lead downwards beneath the lancet- 

 plate into the hydrospiral canal, and are not floored by the 

 oral plates, as are those oi. Pentremites. But the anal spiracle 



* The peculiar manner iu which the spiracles of Schizohldstus ara^ 

 formed is excellently shown iu Wuchsmuth and Springer's figurci oi 

 S. Sat/i (Revision, ii.). But we cannot at all acquiesce iu their in- 

 terpretation of the calycular plates of this type. It is generally supposed 

 to have large orals which form the greater part of the calyx. The above- 

 named authors, however, limit their name to small rhomboid pieces im- 

 mediately round the summit, which we take to be merely the apical ends 

 of large oral plates. Further, they represent a suture as occurring below 

 these tongues and the great plates outside them, which they indicate as 

 the fork-piece (radials). We cannot, however, make out either this 

 sutui-e or the iuterradial one represented by them as between the two 

 limbs of adjacent radials ; for the latter occupies tlie middle line of what 

 we, like earlier writers, take to be large deltoid or oral plates. 



Ann. d; Mag. N. Hist. Ser. 5. l^ol. ix. 18 



