the MorpJiologrj of the Blastoidea. 215 



radial sinus to a greater or less extent, the most important is 

 tlie lancet-plate, wliicli is excavated lengthways by the food- 

 groove or ambulacrum proper. Upon or against it rest the 

 side plates (pore-plates, litt.)^ which thus conceal it more or 

 less completely ; they are marked by minute pits, from 

 which delicate grooves slant outwards towards the marginal 

 pores. The latter are unconnected with the pinnules, which 

 are arranged in a single or double row at the sides of the 

 ambulac]-a. Their bases are apparently inserted into the 

 above-mentioned pits ox jjinnule-sockets. In many species the 

 distal edge of each pore is bounded by an outer side plate 

 (supplemental pore-plate, Hit.). 



Between and more or less beneath tlie ambulacral fields are 

 the interradial systems of lamellar tubes or h^drosjn'res. The 

 openings of these tubes directly on the ventral surface of the 

 calyx, as in Cadaster and Phanioschisma, are the hydrospire- 

 slits. When these organs are concentrated beneath the am- 

 bulacra, the gap between the edge of the lancet-plate and the 

 sides of the radial sinus is the hydrospire-deft ; it leads down- 

 wards into the hydrospire-canal^ into which the hydrospires 

 open by their slit-like upper ends. The hydrospire-cleft is 

 much reduced and somewhat modified in the American species 

 of OropJwcrinuSj but is widely open in the European species, 

 especially in the Belgian ones, so as to expose some of the 

 hydrospire-slits. In Pentremites proper it is also wide, but is 

 bridged over by the side plates, between which are the Itydro- 

 sjnre-jwres. In Granatocritms and Schizoblastus the inner ^vall 

 of each hydrospire, /. e. that nearest the median line of the am- 

 bulacrum, is often carried upwards in certain parts of the 

 ambulacra towards the ventral surface. Here it appears as a 

 narrow plate-like edge between the lancet-plate and the side 

 of the radial sinus. We have seen this hydrosjnre-plafe very 

 distinctly in Schizoblastus melo and in some of the British 

 species of Granatocrinus [O. elJipticus^ G. derhiensis). It 

 bears a number of lateral processes, which meet corresponding 

 ones upon the sides of the radial sinus, so that the hydrospire- 

 cleft is represented merely by a row of pores alternating with 



examhie any specimens of Blustoidocrimis, wliicli we only know from the 

 fio-ures of Billings and Schmidt. But Ave imagine the suture referred to 

 hv Wach.smnlh and Springer to be the suture between the radials and 

 ovalfi {oro-radial), just as is shown in the hypothetical figure given by 

 Billings (' Canadian Decades/ iv. p. 20), in whose interpretiition of the 

 calyx we entirely concur. 



The form of the radials in the Mesozoic PhyUoc'-inm also indicates 

 clearly that the fork-pieces of the Blastoids are prir itively simple and 

 undivided. 



16* 



