170 CATALOGUE OP THE BLASTOIDEA. 



Pentremites (PL IV. figs. 9, 11), or else it approaches in outline to that of Orophocrinus 

 (PI. V. figs. 1, 9, 18; PI. XVI. fig. 11). These apparently dissimilar forms possess 

 in common a slender base, narrow ambulacra, and deltoid plates, which are almost 

 or entirely limited to the summit. The side plates of the ambulacra lie directly 

 upon, and in a great measure cover, the lancet-plates (PI. IV. fig. 12 ; PI. V. figs. 3, 

 4, 19), except in one species (PI. IV. fig. 14). Such characters are diametrically 

 opposed to those of the typical Pentremites, when represented by such species as 

 P. Godoni, Defrance, sp., P. sulcatus, Roemer, and P. pyriformis, Say (PI. I.). 



The external form of the calyx seems to vary more in this than in any other genus 

 of the Blastoidea. We can distinguish two fairly distinct types, the pyriform and the 

 clavate respectively, which are connected by several intermediate gradations. The 

 type species of the genus, P. Paillettei (PI. IV. fig. 9 ; PI. XIV. fig. 13) stands at one 

 extreme of the series ; and there is a gradual transition through P. Lusitanica (PI. IV. 

 fig. 11 ; PL V. fig. 20) to P. Eifelensis (PL V. fig. 1), and P. Boemeri (PL XVI. fig. 11). 

 From these we pass on the one hand to the round-topped species which more or less 

 resemble Orophocrinus inflatus and 0. pentangularis (PL XV. figs. 1, 9), such for 

 example as P. angulata (PL IV. fig. 13), P. Gilbertsoni, P. Malladai, and P. similis 

 (PL V. figs. 9, 18; PL X. fig. 2); and on the other, through the different varieties 

 of P. clavata (PL IV. fig. 18 ; PL V. figs. 3, 17) to P. Wachsmuthi, with its elongated 

 radials and shorter base (PL V. fig. G), and lastly, to such forms as P. ? leda (PL V. 

 fig. 12) which resemble some species of Mesoblastus (PL VI. fig. 12; PL VIII. fig. 1) 

 both in general outline and in the appearance of the deltoids externally. In fact 

 P. \ leda and some of its allies in the American Devonian would have to be transferred 

 to Mcsohlastiis if they should be found to possess the hydrospire-plate, which is so 

 very characteristic of this genus (PL IV. fig. 4 ; PL VI. fig. 10). 



These are exceptional species, however, for the deltoid plates of Pentr ■emit idea are 

 rarely visible externally in a side view of the calyx, being overlapped by the limbs of 

 the radials as in Triccelocrinus (PL XIX. fig. 13) and the other Troostoblastida? 

 (PI. IV figs. 11, 13, 18; PL V. figs. 3, 17, 20). They are just visible, however, in 

 P. Wachsmuthi (PL V. fig. 6) and also in the specimen of P. Paillettei which was 

 figured by Roemer l ; while they are almost equally small in Pentremites calycinus, 

 P. gemmiformis, and even in some varieties of P. pyriformis (PL II. figs. 29, 30). 



The side plates of Pentremitidea angulata do not reach very far over the lancet- 

 plate (PL IV. fig. 14) ; but as a general rule they cover it entirely, not even a median 

 groove being visible on its surface (PL IV. figs. 12, 15 ; PL V. figs. 3-5, 19). Those 

 of P. Lusitanica are of considerable size, and support large side plates which project 

 beyond them and alternate with the pores (PL IV. figs. 12, 15; PL X. fig. 1). 



The complexity of the hydrospire apparatus of Pentremitidea seems to be almost 

 as variable as the outline of the calyx. P. angulata has but few slits (PL IV. fig. 14); 



1 ArcLiv f. Naturgeseh. 1801, Jahrg. xvii. lid. i. T. iv. f. 17 6. 



