Summit Structure of Pentreynites. 497 



cause the necessity of generally regarding the more prom- 

 inent modifications of the inferior parts as family distinc- 

 tions. Indeed in a few cases, even a limited number of 

 the component plates of those parts will indicate as 

 clearly the family as the generic relations. For instance, 

 while a pentagonal, tripartite base of unequal parts will 

 give no indication whether it belonged to a Platycrinus or 

 a Mespilocrinus — two genera belonging to widely dis- 

 tinct families — yet a tripartite base connected with sub- 

 radials indicates with precision, so far as our present 

 knowledge extends, that the specimen belongs to a family 

 which should m^Axxde Forbesiocrinus and Mespilocrinus ; * 

 and a tripartite base supporting five radials and an anal 

 plate, as clearly indicates the specimen to belong to the 

 great family of Actinocrinidce. 



These, however, are exceptional cases, and if modifica- 

 tions of the present system of classification should be pro- 

 posed, we shall doubtless find it as desirable to possess 

 perfect specimens, to enable us fully to understand their 

 family, as their generic or specific characters. 



A recognition of the generic value of the structure of 

 the superior parts of crinoids has been made by authors 

 in the establishment of the genera Bursacrinus, Meek and 

 Worthen, Pterotocrinus, Lyon and Cassady, Trematocrimis, 

 Hall, etc. Going beyond this, Prof. Hall has established 

 two sub-genera — Homocrinus from the Silurian, and 

 Scaphiocrinus from the Carboniferous rocks ; and the la- 

 mented Troost left us the generic description of Zeacrinus 



* These two genera, founded by De Koninck and Le Hon, were not assigned 

 by those authors to the same family; the propriety of which is apparent since the 

 modification o( the formuhi of Forbegiocrhius by Prof Hall. A striking feature 

 common to both these genera is the want of any indication that the upper part of 

 the body was covered with a plated integument, as is the case in most other fossil 

 crinoids, but that these parts were in some cases protected by the folding together 

 of the arms, or their analogues. This peculiarity was doubtless also possessed by 

 kiitli yoc rinus, {dthough its formula is quite different from either of the others; 

 which fact will debar its use as a family character. 



JOURNAL B. S. N. U. 63 JANUARY, 1863. 



