248 Messrs. R. Etliericlge, Juii., and P. H. Carpenter on 



structure of the spiracles. Those of Troostocrimts are at the 

 sides of the proximal side plates, as in Scln'zohlastus, while 

 those of Pentremites are between the proximal side plates and 

 the mouth. The spiracles, more especially those of T. Uneatus^ 

 are intermediate in structure between those of Pentremites and 

 the American species of Oropliocrinus. The distal portions 

 of the ambulacra resemble those of a Granatocrinns, the 

 lancet-plate and its superposed side plates almost completely 

 fillinf^ u]) the radial sinus, so that the hydrospiral pores are 

 small and inconspicuous. Towards the summit, however, the 

 width of the deep radial sinuses increases considerably, while 

 that of the ambulacra decreases a little, and the side plates do 

 not meet the orals. The hydrospiral canals therefore open 

 out into linear spiracular apertures, those of the anal side 

 being nearer the centre than, and quite distinct from, the anus. 

 If the distal portion of the lancet-plate were in continuous 

 instead of interrupted contact with the sides of its radial sinus, 

 T. Uneatus would have the same form of respiratory openings 

 as Orophocrinus gracilis, M, & W. sp., a slit extending for 

 a short distance along each side of the ambulacra. 



On the other hand, if the summit were rounder, the radial 

 sinus shallower,, and the spiracles shorter and wider, Troosto- 

 crinns would be essentially a Pentrcmite with narrower am- 

 biilacra than usual. Therein lies the difference between the 

 two types, and also between Troostocrimts and Pentremitidea. 

 We believe that this form of spiracle, coupled with the chci- 

 racters indicated by Dr. Troost, Avill prove to be of generic 

 value; and we propose to adopt Troostocrimis accordingly. 



The peristome is more contracted in T. Peinwardtii and T. 

 Uneatus than in almost any other Blastoids. The oral plates 

 are very minute, entirely coniined to the summit, as well as 

 inconspjicuous, like those of Pentremitidea and Phcenoschisma. 

 The ambulacra are deeply set in the radial sinuses, but deeper 

 in T. Uneatus than in the other species. The side plates are 

 few in number in T. Peinivardtii, but much more numerous 

 in T. Uneatus. The lancet-])late is almost entirely concealed 

 by the side plates, as in some species of Granatocrinus and 

 HcliizohJastus. In T. Uneatus it contains three canals ar- 

 ranged in a triangle, similar to those of Scliizohlastus. The 

 hydrospires, in the only two forms in which we have seen 

 them, are three or four in number on each side ; T. Reinioardtii 

 possesses three, and T. Uneatus four. 



The following is a full list of the species which will pro- 

 bably be comprised in IVoostocrinus'^: — 



* We are acquainted by examination with the two type species only, 



