126 CATALOGUE OF THE BLASTOIDEA. 



(PI. XVIII. figs. 16, 19), and the presence of an anal plate, which is the distinguishing 

 character of the latter genus, is as it were foreshadowed in the superficial markings 

 on the other deltoids of this type and on those of Schizoblastus Sayi, as we have 

 pointed out on p. 35. There is also a considerable resemblance in the structure 

 of the spiracles between this species and Metablastus Uneatus, different as they are 

 in external form ; while they both possess a triply perforate lancet-piece as shown on 

 PL XVII. figs. 1, 18. 



Metablastus itself passes rather gradually into Tricalocrinus. The extreme forms, 

 such as M. Uneatus and Triccelocrinus Woodmani (PL III. figs. 14, 15 ; PL XIX. 

 figs. 13, 15), are very different; but they are closely linked together by forms like 

 Hall's Pentremites (Metablastus) Wortheni, the Triccelocrinus obliquatits of Meek 

 and Worthen, and our owli Metablastus Meekianus (PL XVI. figs. 17, 18). It is 

 possible that both in the last-named, and also in the Pentremites Grosvenori of 

 Shumard, the deltoids appear in a side view of the calyx as in the Pentremitidge ; 

 while in Troostocrinus Eeimvardti one deltoid only, that of the anal side, is thus 

 exposed. The type which Hambach described as Pentremites clavatus has much the 

 form of a Metablastus, though we should not like to refer it to this type, as the 

 ambulacra are said to join one another above the deltoid ridges. It is very like our 

 Metablastus Hispanicus (PL V. fig. 21), and gives us an excellent transition between 

 M. Uneatus or M. Wortheni and some forms of Pentremitidea (PL V. fig. 17; PL XIV. 

 fig. 13), and also of those species of Pentremites which show but little of the deltoids 

 externally, such as the P. pyriformis represented on our PL II. fig. 30, or Hambach's 

 other species P. gemmiformis. 



The link between Pentremites and Pentremitidea is naturally very close, as will be 

 evident on comparison of our figures of Pentremites conoideus and P. elongatus 

 (PL II. figs. 15, 16) with that of Pentremitidea X leda on PL V. fig. 12. The 

 extremely narrow ambulacra of the last-mentioned type have induced us to refer it 

 to Pentremitidea, and there appear to be some other good species of this genus which 

 also have the deltoids visible on the exterior of the calyx. 



If Pentremitidea 1 leda had an hydrospire-plate and its spiracles more distinctly 

 double, it would be almost indistinguishable from Mcsoblastus Sowerbii (PL V. figs. 

 12-14; PL VI. figs. 12-14) ; and this type again approaches rather closely to some 

 forms of Schizoblastus which differ from it in the absence of an hydrospire plate. 

 The slit-like spiracles of Schizoblastus are simply portions of the proximal ends of 

 the radial sinus which remain open between the narrow ambulacra and the sides of 

 the deltoids. Almost the same may be said of Metablastus Uneatus (PL HI. figs. 14, 

 15), and there is not much difference between the spiracles of this type and those of 

 such a form as Orophocrinus verus (PL XV. fig. 3 ; PL XVI. fig. 10), unlike as the 

 two calyces are in their general appearance. Orophocrinus verus is the nearest 

 approach to 0. stelliformis on the one hand, and on the other it is closely related to 



