100 CATALOGUE OF THE BLASTOIDEA. 



1-3) and in Acentrotremites (PL XIII. fig. ID). Another point of interest about the 

 genus is that the two spiracles in the anal intcrradius always remain distinct from 

 the anal opening, as in the two types just mentioned (PI. XI. fig. 9 ; PI. XIV. fig. 16 ; 

 PI. XV. figs. 3, 8, 11 ; PI. XVI. fig. 10), and do not become confluent with it as in 

 Cryptoblastus (PI. VII. fig. 14), and in most species of Pentremifes, Pentremitidea, 

 and Mesollastus (PI. IV. figs. 1, 14, 17 ; PL V. figs. 2, 4, 7, 10, 13, 15, 16, 19 ; 

 PL VI. figs. 7, 8, 13). 



The structure of the spiracles in Pentremifes may be easily derived from their con- 

 dition in Oropliocrinus, and especially in 0. verus. If the dissected ambulacrum of 

 this species be compared with the corresponding parts of Pentremifes piriformis 

 (PL I. figs. 6, 7 ; PI. XII. fig. 13 ; PL XV. fig. 4) a considerable amount of resem- 

 blance may be traced between the two types. In the latter, as in the former, the 

 proximal ends of the adjacent deltoids approach one another beneath the intervening 

 ambulacrum and form a bed for the lancet-plate ; and a little further out they support 

 the proximal ends of the hydrospire-folds. The slits intervening between these folds 

 open into the bottom of the radial sinus, which is relatively narrow in Orophocrinus 

 verus and incompletely filled by the ambulacrum ; so that a portion of its proximal 

 end is left uncovered at each side of the latter. This constitutes the spiracle, and it 

 is separated from its fellow of the adjacent ray by the body of the deltoid plate which 

 appears on the exterior of the calyx, as is well shown in PL XV. fig. 3 and PL XVI. 

 fig. 10. In Pentremifes, however, the radial sinus is wider than in the typical 

 Orophocrinus, especially at its proximal end, where it encroaches considerably on the 

 bodies of the deltoids at its sides, so that each of these becomes reduced to a mere 

 septum-like ridge. This divides the flattened proximal end of the plate into two 

 equal portions, which thus form the floor of the radial sinus and are only properly 

 seen when the ambulacra are removed (PL I. fig. 6). The sides of these ridges, and 

 in fact of the whole sinus, slope very gently downwards, and are marked by a trans- 

 verse series of alternating ridges and furrows, which were first noticed by Troost x 

 (PL I. figs. 5-7 ; PL XII. figs. 13, 16). They are not mentioned at all by Roemer 2 , 

 in whose figures the sloping sides of the radial sinus are represented as perfectly 

 smooth ; but they were described by Hambach ? ', who gives a good figure of them. 

 The furrows receive the narrow outer ends of the side plates, the broader inner ends 

 of which simply rest against the edges of the lancet-plate without covering any part 

 of it (PL I. figs. 5, 6 ; PL XII. figs. 13, 14 ; PL XVI. fig. 21). As the lancet-plate only 

 occupies the middle of the radial sinus there is a wide space left on either side of it 

 which leads outwards above the hydrospire-slits. This corresponds to the whole 

 spiracle of Orophocrinus (PI. XIV. figs. 16-18; PL XV. figs. 4, 8-10); but the 



1 Trans. Geol. Soe. Pennsylvania, 1835, vol. i. p. 227. 



2 Archiv f. Naturgcsch. 1851, Jahrg. xvii. Bd. I. Taf. iv. figs. 2, 3. 



3 Trans. St. Louis Acad. Sci. 1880, vol. iv. no. 1, p. 147, PI. A. fig. 2. 



