OAvn:>ii>i>\ 



Trjrblldlum ) 



that those pores are really a normal feature of the external layer receives strong 

 -npport from the fa<-t that a porous outer layer occurs also in other patelliform 

 shells. We have, namely, specimens before us of two shells belonging to an unde- 

 srril>t><l L'emi- in which the whole external surface is beautifully punctate. One of 

 the-e -pecimrn- U from tin- top ( if tin- I'iiirimiati jrmup at Richmond. Indiana, t lie 

 other from the middle Devonian at the falls of the Ohio. 



1'nif. .1. K. Whiteaves, paleontologist to the Geological Survey of Canada, was 

 the first to recognize the application of TrylHdiuin to American fossils. In referring 



.inadiair patelliform shells (<>j>. rit.. p. 31), he subscribes to the suggestion of Mr. 

 hall < . \nit-r. .louni. ('ouch., vol. vi, p. 281, 1881) that none of the nineteen -pecies 

 provisionally referred to Phillip-' genus Mefoptoma really belong to that genus as 

 now understood. Continuing he says that in his judgment " M>i<if>in>na quebecensis 

 Hillings, lielongs to the genus r<il<iaci'in of Hall and Whitfield. M. niobe, M. nycteis, 

 M. fubulr, M. tntfo and M. hyrie Hillings, are typical species of Trylilidium, Lindstrom." 

 So far he expresses our views exactly, but when it comes to the new species, which 

 he names T. ranmlense, we, as did also Lindstrom in his second work on the genus (p. 

 54), note differences that necessitate its removal from TryMiilium. To us it is an 

 undoubted member of our new genus Arclnnacellu. 



In 1S$<; Prof. H. I'. Whitfield (op. cit.) described two species, ovale and ovafmn, 

 which are unquestionably congeneric with T. unguis Lindstrom. In his remarks on 

 the species he points out some differences which exist between his species and the 

 generic description given by Lindstrom, and suggests that some of the characters 

 mentioned in the latter are only specific and not generic. This is true of the 

 aperture so far as its being straight or arched is concerned, but we cannot agree 

 with him when he places in the genus species like his T. conicum, in which the apex 

 is almost central. Such species should in most cases be regarded as belonging to 

 Srentlla or Pakracmcea. Further, he notes a difference in the number of the 

 muscular scars, Lindstrom stating that the Gotland species have only six pairs, 

 while his species have eight pairs. Again, he found that his species differed from 

 LindctrOm't generic diagnosis in having the muscular scars "continuous around and 

 below the apex of the shell, in a deep ami continuous line, from the elongated 

 clavate scars on the sides of the beak or apex, as they are in Nacelfa" instead of 

 "open or nearly so toward- the outer end." 



A careful study of 7'. um/i//.>, which we owe to the kindness of hr. Lindstrom 

 himself, enables us to show that there is no essential difference between the niu-cnlar 

 scars of this Gotland type of the genus and those shown to exist in the i ort 

 Cassin beds species by Whitfield. The fact i- that '/'. //wyu/v has eight instead of 

 six pairs of scars, and the narrow ends of the anterior pair, which is smaller than 



