ItKSCKII'TloN'S ()!■' TIIK SI'I'CIKS. 



193 



and its allies from the Carboniferous strata. One of these later forms, however, 

 /'. Qrosvenori, Shumard, should perhaps be referred to Trooatocrinua as we define it , 

 for it lias five undivided spiracles, one of which is confluent \\ith the anus, as is well 

 seen in Shumard's figure of the summit. He described the deltoids of this type as 

 " small, lozenge-shaped, and rather prominent in the middle;" and in the side view 

 which he figures one deltoid answering to this description does appear externally '. 

 It may also be traced, though not clearly so, in the figure of the summit; but the 

 interradial sutures in the other four interradii appear to come right up to the 

 spiracles. If this he really the case, Pentrermtes Grosvenori is an undoubted Troosto* 

 crinns, and the genus will then range up into the Carboniferous period. It seems 

 to be a rare species, and the question of its generic position can only lie solved by our 

 American colleagues. We have no information respecting any oilier specimens than 

 the one described by Shumard in 1S5S. 



Pic. VII. 



Diagrams of Troostocrinw Beinwardti. A. The anal side. IS. Summit view. The 

 radio-deltoid sutures are shown by rather darker lines. 



Remarks. The genus Troostocrinus, as limited by us, is one of peculiar interest. For 

 it is the only Silurian genus of Blastoidea which is at all satisfactorily known, and 

 it presents a curious departure from the ordinary structure of the regular Blastoids 

 in having the deltoid of the anal side so much larger than its fellows as to appear 

 on the exterior of the calyx above the radial limbs (Fig. VII.). The only other 

 exceptional structure of this kind which we know of in the regular Blastoids is the 

 division of the posterior deltoid of Elasacrirms into two parts by an anal plate 

 (PI. XVIII. fig. 19). This is foreshadowed, as it were, in the superficial division o\ 

 the other deltoids both in this genus and also in Schizoblastus Sayi, as we have pointed 

 out on p. 35; and in like manner the anal deltoid of a specimen of Pentremitidea 

 Paillettei was noticed by Koemer 2 to be larger than its fellows, which are just visible 

 externally, though this is not the case in Troostocrinus, as the limbs of the radials 

 completely overlap the four anterior deltoids so that only their inner faces are 

 exposed (Fig. VII.). 



1 Trans. St. Louis Acad. Sri. 1,N>, vol. i. no. 1', p. 241 , pi. !.). tigs. 2 a, 26. 

 - Arehiv f. Xaturgcsch. 1851, Jahrg. xvii. Bd. i. p. 3G9, Taf. iv. fig. 17 b. 



2c 



