NO. 1668. A NEW GENUS OF UNSTALKED CRINOJ DS— CLARK. 353 



laterally; that on the anal side bears a short jointed appendage; 

 mouth central and protected by five large oral plates which occupy 

 the greater j^art. of the disk, and are separated from the calyx inter- 

 ladials by two or three rows of small irregular plates; five arms only. 



Doctor Carpenter discusses at considerable length the similarity 

 between Thaumatocrinus and a number of palaeozoic forms, and, al- 

 though he includes it in the "Comatulidse'' as understood by him, he 

 is inclined to regard it as a very anomalous type, exhibiting certain 

 atavistic characters. The tyi^e-specimen of Thaumatocrinus renova- 

 tvs is exceedingly small ; the total width of the calyx across the disk 

 is barely 2 mm., and the height of the centro-dorsal and radials to- 

 gether is about the same. 



The arm structure of Thaumatocrinus is identical with that of 

 Pentametrocrinns and Decametrocnnus., which together form the 

 family Pentametrocrindae, and in its pinnule and cirrus structure, 

 in so far as it can be made out, it also agrees with the conditions found 

 in those genera. The arm and pinnule structure of the Pentametro- 

 crinidaj is unique in its simplicity among the comatulids, which in 

 itself suggests that it is a family of a remarkabl}^ primitive type. 

 The adult Pentametrocr'inus differs from the adult xlntcdon in its 

 more generalized and presumably more primitive structure; if we 

 take a young Antedon and generalize it by supplementing its single 

 interradial (anal) with four others like it, we would find a Thauma- 

 tocrinus as a result. ThauTnatocrinus in every detail except such 

 as, from analogy with Antedon, may be safely ascribed to immatu- 

 rity, agrees with Pentametrocrinus ; hence it seems probable, as the 

 atavistic Thaumatocrinus bears a very similar relation to the young 

 Antedon to that which the primitive Pentametrocrinus does to the 

 adult Antedon, and as Thaumatocrinus and Pentametrocrinus agree 

 in all essentials fully as well as the young and the adult of Antedon 

 agree, that Thaumatocrinus is in reality nothing more nor less than 

 the young of Pentametrocrinus. But there is still further evidence. 

 In Decametrocrinus, which is a meristic variation from Pentametro- 

 crinus, differing only in having twice as many radials and arms, the 

 ends of the five basal rays appear externally in the angles of the 

 calyx, dividing the ten radials into groups of two; but in Proma- 

 chociinus, which is a similar meristic variation from Ileliometra 

 or a closely allied genus, the ends of the basal rays appear exter- 

 nally under the center of alternate radials. Promachocrhwts prob- 

 ably has young much like those of the closely allied Heliometra, in 

 Avhich there is but a single interradial plate, the anal; now the divi- 

 sion of each radial in the young of Promachocrinus, and the growth 

 of each resultant half to the same size as the single undivided radial 

 in Heliometra, would, as the anal plate is lifted out from between 

 the posterior radials, produce a certain amount of tortion of the 



