INVERTEBRATES. 149 



convoluted plate, connected with the digestive apparatus. For 

 an illustration of this organ, as seen in the genus Strotocrinus, 

 see a wood cut given in another part of this volume, in connec- 

 tion with the description of that genus. 



The foregoing description is drawn up so as to exclude some types usually in- 

 cluded in this genus, but which, we think, should be separated as distinct 

 genera. Even as here defined, however, it is susceptible of division into several 

 sections and subordinate groups, as follows : 



t. ACTIIVOCKINirs, Milu.b, (proper.) 



a. — Groups of arm bases, forming five distinctly separated prominent lobes, some- 

 times including the adjacent parts, in to the third primary radial pieces; 

 second primary radials nearly alwaj-s hexagonal ; vault provided with an 

 elongated sub-central proboscis, with the opening at its summit ; body 

 plates generally ornamented with radiating Cjostae. 

 Actinocrinus Iriacontadactylus, Miller ; A. dorsatus, A. stellaris and A. armatus, 

 de Koninck ; A. Lowii, Hall ; A. concinnus and A. multiradiatus, Shumard? 



6. — Differs from the above in having the arm bases not projecting in lobes, and 

 much less distinctly in groups, or sometimes forming a nearly continuous 

 series. 

 A. proboscidialis, A. clarus, A. sezarmatus, A. costatus and A.multibrachiatus, 

 Hall ; A. Hurdianus, McChesney ; A. polydactylus, Miller, etc. 

 c. — Agrees with the last, excepting in having the sub-central opening of the vault 

 simple or without a' proboscis.* 

 Actinocrinus ornatus, A. cancellatus and A. ventricosus, Hall; A. subventricosus, 

 McChesney. 



* We are in considerable doubt in regard to the nature and importance of the dif- 

 ference between species with a simple opening, near the centre of the summit, and 

 those provided with an elongated central or subcentral proboscis, with the opening 

 at its extremity. At one time we were inclined to think this a character of considerable 

 importance, indicating fundamental differences in the structure of the animal ; but 

 the fact that we find this difference occurring in various genera of palaeozoic crinoids, 

 even after being carefully restricted and grouped upon all other characters, has led us 

 to doubt its value as a generic distinction, and even to suspect that it might have been 

 in some cases a sexual difference, since the Echinodermala are known to have the sexes 

 distinct. Yet the specific relations of the forms thus distinguished do not seem to 

 sustain this supposition; while the well-known fact that the reproductive organs in 

 the recent Crinoidea are not located within the body, but on the tentacles of the 

 arms, renders it very improbable that the presence or absence of a proboscis could 

 have depended upon the sex of the animal. In a few rare cases species like Actinocri- 

 nus oblatus, Hall, which have a very slender proboscis, seem to have had that appen- 

 dage sometimes accidentally broken away, during the life of the animal, so that the 

 fractured edges of its base become absorbed and round in such a manner as to present 



