112 TRANS. OF THE ACAD. OF SCIENCE. 



them in the genus Ekeacrinus of the same author. We there- 

 fore propose now to group in the latter genus such forms as 

 Pentremites melo, P. Norwoodi, P. curtus, P. gramdatus, 

 P. JRcemeri, P. Sayi, P. cornutus, and the species we are 

 about to describe; also the Niideocrinus annularis of Lyon. 



Among European species, the following may be grouped in 

 this genus : Pentremites ellipticus, P. orbicularis, P. Derbi- 

 ensis, P. oblongus, and P. angulatus. 



These form a very natural group, easily recognizable, and 

 distinguished from the typical species of Pentremites by well 

 marked characters. They are always of an elliptical or sub- 

 globular shape. The pseudo-ambulacra! areas are narrow, 

 with sides subparallel, and extend usually the entire length 

 of the body. The basal pieces are nearly always concave, 

 and generally situated at the bottom of a deep excavation. 

 The tubular lamellae, which in the interior reach from the 

 ovarial apertures to the base of the pseudo-ambulacral fields, 

 are more simple in structure, being much less convoluted, 

 while the relative position of the ovarial apertures is dif- 

 ferent. 



The typical species of the genus Ekeacrinus, viz., E. Ver- 

 neuili, long previous to the publication of Prof. Roemer's de- 

 scription with figures was well known to American and also 

 to some European pakeontologists as Olivanites Verneuili, 

 under which name it was designated by Prof. G. Troost in 

 his Monograph on North American Crinoidea, which valua- 

 ble memoir was completed a short time previous to the death 

 of its author, but is not yet published. According therefore 

 to the laws of priority, Roemer's name, Elmacrinus, proposed 

 in 1852, must be adopted, although it is to be regretted that 

 the learned author did not adopt Troost's generic name in 

 preference to creating a new one. 



It is possible that the genus Niideocrinus, proposed by 

 Conrad in 1842, (Jour. Acad. Nat. Sci., Philad., vol. viii., p. 

 280, pi. xv., fig. 17,) may be identical with Elmacrinus ; but 

 the meagre and unsatisfactory description of Conrad ( " this 

 genus differs from Pentremites, Say, in having only one per- 

 foration at top, which is central") does not apply to any of 

 the forms we propose to group in Eheacrinus. 



The American species hitherto discovered are distributed 

 through the Geological formations as follows : In the St. 

 LouisLimestone two, E. Kirkwoodensis and E. cornutus ; 

 in the Warsaw Limestone one, E. curtus ; in the Encrinital 

 Limestone three, E. Norwoodi, E. melo, and E. Sayi ; in 

 the Chemung Group one, E. Rc&meri ; and in the Devonian 

 two, E. Verneuili and E. angularis. To these we may add 

 an undescribed species found by the writer in the Silurian 

 (Upper Ilelderburg) strata in Perry county, Missouri, and 

 now in the Missouri State Collection. 



