88 CATALOGUE OF THE BLASTOIDEA. 



genus are altogether unknown. Those of Heteroblastus are quite small (PI. VI. 

 figs. 3, 4), and nothing is known of its hydrospires except the internal markings upon 

 the radials and deltoids. The hydrospires of Acentrotremites are also unknown, but 

 we may infer that they are normal from the fact that the spiracles and hydrospire- 

 pores are tolerably similar to the corresponding parts of Cryptoblastus (PL VII. 

 fig. 15 ; PI. XIII. fig. 19). In all the other Blastoid genera, however, we have been 

 able to obtain a personal knowledge of the hydrospires, sometimes in two or more 

 species, and in most cases also in very different states of preservation. Their struc- 

 ture will probably be most clearly understood if we commence by studying them in 

 Codaster and its allies. 



In the flat-topped species of Codaster which occurs in Britain, four of the inter- 

 radial areas on the summit of the calyx are marked by a series of slits with inter- 

 vening ridges (PI. XIII. figs. 1, 4, 8). They lie parallel to the ambulacra, and cross 

 the radio-deltoid sutures nearly at right angles. The slits are actual breaches of 

 continuity in the substance of the calyx-plates (PL XIII. figs. 6, 7) ; they pass 

 through the thin lateral portions of the deltoids, and also through the horizontally 

 depressed upper parts of the radials (PL XII. fig. 8; PL XIII. fig. 7). All that is 

 left then of the ventral surfaces of these plates is represented by the ridges between 

 the hydrospire-slits. Each of the latter leads downwards into what has been well 

 described as a " lamellar tube." Two of Rofe's sections showing respectively the 

 upper and lower portions of these tubes are seen in PL XIII. figs. 5, 7, the first 

 of which represents a section along the north-western face of the rubbed-down calyx 

 shown in fig. 7 ; while the relations of the tubes to the body-cavity are still more 

 clearly seen in the weathered specimen shown in fig. 6. The whole of one radial 

 has been removed, thus exposing the tubes which dip inwards towards one another 

 beneath the ambulacrum, as is also seen in PL XVIII. fig. 1. 



In the American species of the genus the calyx is much less flat-topped than in 

 Codaster trilobatus. The radials are no longer so wall-sided, and show traces of a 

 division into body and limbs. The oral crests thus become prominent and the 

 ambulacra lie in sinuses between them as shown in C. pyramidatus, C. Hindei, and 

 also in C. alternatus, var. elongatus (PL XII. figs. 1-6 ; PL X. figs. 19, 20). The 

 radio-deltoid sutures descend the sides of these sinuses, which are pierced by hydro- 

 spire-slits just as in C. trilobatus, and though as in that species (PL XIII. fig. 14 ; 

 PL XVIII. fig. 1) one or more of the slits may be concealed by the projection of 

 the side plates at the edges of the ambulacra, the majority of the slits are freely 

 exposed and open directly to the exterior. 



The same is the case in some species of the genus Pluenoschisma, which differs 

 from Codaster in the presence of hydrospires in the anal interradius, so that there 

 are ten groups of hydrospire-slits instead of only eight. The little P. Benniei has 

 but a few large and widely separated slits (PL II. fig. 37). But in P. nobile and 



