ICHTHYOCRINIDAE 305 



with intervals of one ; the less characteristic species, D. alpena and D. tardus, have intervals 

 of 4 and 5. In these two species the heterotomy is of an intermediate character, the ramnles 

 becoming enlarged to more nearly the size of the main arm trunks ; the last named is the 

 latest survivor of the genus. If the mode of arm branching were to be taken as of control- 

 ling weight in the arrangement of families, Dactylocrinus and its heterotomous allies would 

 all be placed in one family, as Bather has done. This arm structure seems to me a subordinate 

 modification which is repeated independently in several well-defined families of both the 

 other orders of the Crinoidea, viz., Dimerocrinidae, Melocrinidae, Actinocrinidae, Platy- 

 crinidae and Hexacrinidae of the Camerata, and Cyathocrinidae of the Inadunata. Accord- 

 ing to my view, a parallel modification occurs in most of the families of the Flexibilia, 

 although without any correlation as to geological succession. 



In the basal structure the characteristic of the family is strongly marked, with a ten- 

 dency of the infrabasals to retreat still further from participation in the calyx wall into a 

 greater concavity ; this reaches the extreme in D. excavatus, where they are buried at the 

 bottom of a deep excavation involving the basals and part of the radials, and filled by the 

 column for a considerable distance (PI. XLI, fig. $d). The form and proportions of the 

 basals are remarkably constant in the genus and constitute one of the best characters for 

 recognizing it ; they usually project from the concavity in more or less long, narrow tongues, 

 but in every case the posterior basal rises high above the rest to nearly or quite the upper 

 margin of the radials. 



In superficial appearance the resemblance between Dactylocrinus and Synerocrinns is 

 very strong, but the difference in the construction of the anal area is decisive ; that of 

 Synerocrinus being a single vertical series of plates soon to become a tube bordered by 

 perisome, while in Dactylocrinus the anal plates are in more than one series, filling the lower 

 part of the area by sutural union with the brachials; and the anal x, instead of being a 

 quadrangular plate, is angular above, supporting two or more plates. The difference noted 

 by Bather (Lankester's Zoology, pt. 3, p. 190), that in Dactylocrinus the proximal ramule 

 branches again seems not to be very reliable. In Pacht's description and figure of D. oligop- 

 tilus this appears to be so at only one place, whereas the nineteen other proximal ramules are 

 of a simple type. In Synerocrinus the interbrachial system is more profusely developed, 

 mature individuals having five to eight plates, and even young specimens have more than one ; 

 while in Dactylocrinus the number is rather constantly limited to one, or sometimes none. 

 The two genera are of widely different geological age, and appear to be closely limited to 

 their respective horizons. 



Another genus very similar to this in arm structure is Wachsmuthicrinus, which follows 

 it closely in geological succession, and might well be taken as the successor of Dactylocrinus, 

 with all anal structures eliminated and to some extent the interbrachials also ; the latter are 

 rarely absent in Dactylocrinus, but frequently so in Wachsmuthicrinus. By some confusion 

 of types the analysis of genera on page 518 of my paper of 1906 (Jour. Geology, vol. 14), 

 was made to state the number of main trunks in both these genera as 10, instead of 20. 



For a type so little known in the first instance the literature of the genus is involved in 

 an unusual amount of confusion. It was described by Pacht in 1852, from specimens from 

 the Upper Devonian limestone of the government of Pskov, Russia, under the name Dimero- 

 crinites oligoptilus, with very fine and elaborate illustrations. He assigned his new species to 

 Phillips's genus under a complete misapprehension of the characters of the latter, which 

 Phillips had proposed for species which he declared himself to be " unable, at present, to 

 characterize completely." Having the impression from certain remarks of Johannes Miiller 

 and Phillips that the much discussed species, Potcriocrinus uobilis Phillips and Cyathocrinites 

 tuberculatus Miller, did not belong to their respective genera, and recognizing in his own 

 species a certain resemblance to them, Pacht concluded to place all three under the still less 

 understood Dimerocrinus, making P. nobilis and C. tuberculatus synonyms under the name 



