XO.2038. FOSSIL CRINOID GENUS H0M0GRINU8—KIRK. 479 



HOMOCRINID.^, new family. 



Monocyclic Inadunata, with 5 B B, 5 R R (3 compound) and an 

 anal x in the cup. The 1. post. R and ant. R are large and undivided. 

 In the other three rays the radials are compound, x enters into the 

 composition of the cup, resting equally on the right shoulder of 1. 

 post. R and the left shoulder of r. post. R. The presence of an anal 

 tube is predicated. The arms are nonpinnulate and do not bifurcate. 



The family as here defined mcludes but the one genus Homocrinus. 



Whether Homocrinus itself ever gave rise to a line of descendants 

 is a question at present impossible of solution. The Homocrinidae 

 or forms of very similar structural character might perhaps serve as 

 the ancestral stock for the Pisocrinidse, Haplocrmidse, and similar 

 types. A crinoid not widely divergent from Homocrinus might, on 

 the other hand, have formed the radicle from which sprung the Heter- 

 ocrinidae. Such modifications as are to be observed in these genera 

 are no greater than one might reasonably expect, and indeed the only 

 types from which the Pisocrinidse and Haplocrinidje could be derived 

 would partake very largely of the nature of Homocrinus. One can 

 but hope that future collections will make it possible to work out in 

 some detail the evolution of this or similar minute forms. With this 

 data in hand the bearing such types have on the evolution of the 

 Crinoidea in general will become more obvious. Until such time our 

 conclusions though apparently logically sound can be but specula- 

 tive at best. 



The existence of such a form as Homocrinus parvus causes one to 

 wonder if during Paleozoic time, and perhaps later, there did not 

 live many equally minute crinoids. We know in the Mississippian 

 for example, that there was a species of AUagecrinus quite as small 

 as Homocrinus parvus. Such types were not derived from crinoids 

 larger than themselves. In the development of any group of inverte- 

 brates the trend of evolution is from the small to the large, and never 

 the reverse except in special cases of degeneracy or dwarfing. Homocri- 

 nus though showing no positive tendencies in any direction, owing to 

 our lack of knowledge relative to its ancestors or descendants, cer- 

 tainly shows no signs of degeneracy. The improbability of its having 

 been dwarfed has been noted elsewhere. Such being the case we may 

 postulate for Homocrinus ancestors of similar size or even smaller. 



If we assume the existence of such a practically unknown congeries 

 of microscopic crinoids, as we well may be justified in doing, it seems 

 possible that these small types at various times may have furnished 

 points of inception for evolutionary lines among the Crinoidea. 

 Certain it is that the maintenance of such a basic stock would serve 

 to explain the presence of many otherwise anomalous forms in our 

 Paleozoic rocks. Many of our Inadunata might be cited as examples. 

 Some of these types appear quite suddenly, and though frequently 



