NO. 1846. ON CERTAIN ELEUTREROZOIC PELMATOZOA—KIRK. 101 



It is probable indeed that this "swarming" was the actual cause of 

 the death of tlie crinoids, as has been suggested as a possibihty by- 

 Springer, Setting aside the theory that the crinoids hved in more or 

 less agglomerated masses it seems scarcely possible that they could 

 have lived in freely swimming schools, as do the living Comatulse. 

 Arguing against the li\ang of any considerable number of Vintacrinus 

 in juxtaposition, one needs but to consider their great size. An 

 adult fidly expanded would cover an area of 30 or more square feet. 

 Allowing for a reasonable amount of clear w^ater on all sides, say 

 a couple of feet, we would have the individual's space requirements 

 greatly increased. Under these conditions it seems as if it would 

 prove inexpedient for any considerable number of individuals to 

 travel together. 



There seems but one plausible explanation to account for the 

 "swarming" of Vintacrinus and that has been put forward by 

 Springer. He says: "It is possible that this may have happened 

 during periods of sexual activity." We can easily conceive, under 

 such conditions, of large numbers of Uintacrinus gathering together 

 in favorable localities, such as bays or lagoons. Here possibly 

 through the crowding of females about the males ^ a large number of 

 individuals might become inextricably tangled together. Under 

 such circumstances the individuals constituting the core of the mass 

 must inevitably die of suffocation and starvation, though probably 

 the former course may be held as the more efficient inasmuch as the 

 whole incident could have occupied but a short time. Some of the 

 incUviduals on the outer surface of the mass might be able to clear 

 themselves by a breaking off of their arms, but the majority would 

 be killed. Subsequent to the death of the animals the mass would 

 sink to the bottom, and there form such lenses as we find to-day. On 

 this hypothesis one may understand the absence of isolated specimens 

 in this region. The crinoids came into the shallow waters during 

 periods of sexual activity and afterwards returned to their normal 

 habitat, perhaps far out in the open sea, leaving behind only an 

 occasional mass of their unfortunate fellows. 



A glance at a paleogeographic map of the Niobrara sea gives con- 

 siderable support to the foregoing conclusions. The ancient Gulf of 

 Mexico occupied in a general way the area it now covers, although it 

 was considerably greater in extent. Passing northward into Kansas, 

 however, was a great embayment. In the Gulf itself there was prob- 

 ably a strong Gulf stream much as at the present time. Out in this 

 current, and probably farther out at sea also, the Vintacrinus spent 

 the greater part of the year. In the breeding season they entered the 



1 The prepionderance of females oyer males seems to be quite marked in recent forms. Among nearly 

 thirty specimens of Actinometra japonica collected at a single locality at one time, there were no males. 

 The females in all cases bore large numbers of nearly ripe ova. At the same locaMty A ntedon macrodiscus 

 likewise showed a very marked preponderance of females over males. 



