30 CATALOGUE OF THE BLASTOIDEA. 



(PL V. tig. 12), Mesoblastus elongatus, and M. angulatus (PL VIII. tigs. 1, 7), the 

 deltoids take but little share in the formation of the narrow radial sinuses ; and the 

 greater part of the length of the hydrospires is supported by the radials, the deltoids 

 appearing externally as small plates, or being absent altogether from the exterior of 

 the calyx as in Pent remit idea (PL IV. figs. 9-14, 17, 18; PL V. figs. 1, 3, 6, 

 9, 17, 20). In other types, again, the deltoids are also entirely confined to the 

 summit, and do not appear at all upon the exterior of the calyx, but the radial sinus 

 is wide and nearly filled up by the ambulacra as in Cryptoschisma Schuhi (PL V. 

 figs. 23, 24 ; PL XIII. fig. 20). In Phcenoschisma, again, owing to the great length 

 of the radial limbs the sinus is likewise deep, though it is but very imperfectly filled 

 by the ambulacra. A good instance of this condition is afforded by Phcenoschisma 

 nobile, in which the radio-deltoid suture starts from the ventral end of the long 

 interradial suture, and descends the sloping sides of the radial sinus towards the 

 ambulacra (PI. XI. figs. 1-3). The proximal or ventral part of the sinus in this and 

 other species of the genus (PL XIV. figs. 9, 11, 12) is thus bounded entirely by the 

 deltoid plates just as in Cryptoschisma (PL V. fig. 24). 



In Codaster pyramidatus and C. Ilindei there is a shallower and more open sinus, the 

 walls of which are much less steep than in Phcenoschisma, but traversed in the same 

 way by the radio-deltoid sutures (PL XII. figs. 1-6) ; while in C. trilobatus the radials 

 have no external limbs and the sinus is therefore non-existent, though the positions 

 of its walls are indicated by the two triangular spaces on the flat summit in which 

 the hydrospires are lodged (PL XIII. figs. 1, 4, 8), one on each side of the ambu- 

 lacrum. Each of these is divided into two parts by the radio-deltoid suture. These 

 sutures start from the middle points of the sides of the pentagonal cup so as to mark 

 out a smaller and inscribed pentagon, the whole of which is composed of the five 

 deltoid pieces surrounding the central mouth, just as in the slightly more concave 

 summit of Cryptoschisma (PL V. fig. 24). 



The genus Heteroschisma has been proposed by Wachsmuth l for the reception of 

 three species which appear to him to differ from Codaster in the relation of the 

 hydrospires to the interradial plates. We have pointed out 2 that Phcenoschisma is 

 distinguished by the presence of hydrospires in the anal interradius, and by the 

 relatively small size of the interradials (PL XI. fig. 3 ; PL XIV. figs. 5, 11), while, 

 according to Wachsmuth, Heteroschisma has small interradials like Phcenoschisma, 

 but resembles Codaster in having no hydrospires in the anal interradius. By this 

 latter character, in our opinion, it is sharply distinguished from Phcenoschisma; and 

 we are here quite in accordance with Wachsmuth, who further tells us that he 

 considers " the structural differences in the so-called ' oral plates ' morphologically as 

 important as the difference in the number of the hydrospires." The differences 



1 Report Geol. Survey Illinois, 1883, vol. vii.p. 252; Proc. Davenport Acad. Nat. Sci. Lss.j, vol. iv.p. 82. 



2 Ann. & Mag. Nut. Hist. L882, vol. ix. p. 227. 



