Casey — Notes on the Pleurotomidae. 139 



majority of them have the anal sinus situated on a posterior 

 declivous or concave fasciolar surface as in Surcula and re- 

 lated genera, but, in some instances, as the very isolated 

 Glyptotoma, the sinus is medial and formed on a prominent 

 periphery. This peculiar group of genera is entirely and long 

 since extinct, except CocJdesinra, which appears to have a 

 history almost as extended as Gemmula from the Eocene to 

 the present time, and Aforia, Aniiplanes and Megasurcida, 

 which as far as known are exclusively living. It is the only 

 group containing reversed or sinistral shells, a character which 

 seems to be of generic value, as I have never seen a dextral 

 specimen of Sinistrella americana Aid., among a very large 

 number examined, although otherwise Sinistrella is rather 

 closely allied to Trypaiwtoma, a very distinct genus founded 

 by Cossmann upon the Pleurotoma terebriformis of Meyer. 

 It is also the only group in which the plications of the col- 

 umella become in any way a conspicuous feature, although 

 this character does occur in some of the Surculid genera in a 

 less developed degree ; it is greatly developed in Eucheilodon, 

 ticohinella and Glyptotoma. In Clinura and Cochlespira the 

 whorls are broadly expanded into a thin spiral plate usually 

 reflexed and crenulate at the edge. In Aforia circinata Dall, 

 this expansion is reduced to a small but abruptly formed 

 median ring. 



Euclieilodoii Gabb. 



This genus is abundantly distinct from Scohinella in having 

 the anal sinus formed upon an elevated and prominent peri- 

 pheral shoulder well above the middle of the whorls, and not 

 in a concave posterior fasciolar area ; it also differs materially 

 in the embryo, which, although of the same general multi- 

 spiral type, is very much larger, and in the form of the outer 

 lip, which does not have the broadly lobed and advanced 

 form of Scohinella, in its very narrow linear aperture with 

 more strongly lyrate outer lip and in possessing a columella 

 fold near the posterior end of the inner lip, which is never 

 present in Scohinella. The system of columella folds is 

 more elaborate than in that genus and the spiral lyrae are 



