on British Palceozoic Crinoids. 291 



Mr. Sladen's. Whether the P. tenuis^ Austin, is identical 

 with P. tenuis, Miller, does not concern us now ; but under- 

 basals are represented in the type-figures of both authors ; 

 while in the Messrs. Austin's figure of P. isacohus there is no 

 trace of these plates, and the statement as to their presence is 

 a very guarded one. For the same reason we cannot follow 

 Messrs. Wachsmuth and Springer* in referring P. isacohus, 

 Austin, to the sectional group ScapMocrinus, Hall. We be- 

 lieve its calyx to be a simple one, consisting of a monocyclic 

 base supporting five large radials, just as in our own fossil 

 [Allagecrinus) ; and we are disposed to regard the two types 

 as congeneric, but as specifically distinct from one another. 

 In A. Austinii some (1 to 4) of the radials may be axillary, 

 bearing arms directly without the intervention of any second 

 or thii'd radials. These arms may have divided (but there is 

 no evidence of their having done so) before the fourth joint 

 above the radials. In A. isacohus, on the other hand, there 

 seem to have been but five primary arms, which forked on the 

 third and then again on the fourth joints above the radials. 

 Without a personal examination of the Messrs. Austin's ori- 

 ginal specimen we are naturally unable to say much about it ; 

 but we venture to think we are correct in referring it to a 

 type which is much more closely allied to Allagecrinus than 

 to Poteriocrinus , Dactylocrinus, or Scaphiocrinus. 



4. Position of Allagecrinus with respect to other Families of 

 Crinoidea. 



If we are right in believing that the various forms figured 

 in Pis. XV. and XVI. are merely different stages in the deve- 

 lopment of one singularly protean species, Allagecrinus must 

 be regarded as a type of singular interest ; for although it is 

 a Palaeozoic Crinoid, the most advanced individuals are entirely 

 devoid of those characters which are supposed to be specially 

 distinctive of the Tessellata. According to Miiller's defini- 

 tion of the " Crinoidea articulata," Allagecrinus is as much 

 an articulate Crinoid as Pentacrinus. On the other hand, the 

 younger specimens are truly " tessellate," and they retain 

 the peculiarities which are su])posed to be eminently charac- 

 teristic of the Tessellata until they are considerably larger 

 and more strongly built than the largest Comatula-ldiXYSii. in 

 the " tessellate ^' stage. 



Were we dealing with these specimens only, Allagecrinus 



* Revision of the Paljeocrinoidea, part i. p. 113, extracted from the 

 'Proceedings of the Philadelphia Academy of Natural Sciences/ Nov. 4, 

 1879. 



