GASTROPODA. 825 
Tryblidium.] 
that these pores are really a normal feature of the external layer receives strong 
support from the fact that a porous outer layer occurs also in other patelliform 
shells. We have, namely, specimens before us of two shells belonging to an unde- 
scribed genus in which the whole external surface is beautifully punctate. One of 
these specimens is from the top of the Cincinnati group at Richmond, Indiana, the 
other from the middle Devonian at the falls of the Ohio. 
Prof. J. KF. Whiteaves, paleontologist to the Geological Survey of Canada, was 
the first to recognize the application of Tryblidium to American fossils. In referring 
to Canadian patelliform shells (op. cit., p. 31), he subscribes to the suggestion of Mr. 
Dall (Amer. Journ. Conch., vol. vi, p. 281, 1881) that none of the nineteen species 
provisionally referred to Phillips’ genus Metoptoma really belong to that genus as 
now understood. Continuing he says that in his judgment “ Metoptoma quebecensis 
Billings, belongs to the genus Palewacmea of Hall and Whitfield. M. niobe, M. nycteis, 
M. eubule, M. erato and M. hyrie Billings, are typical species of Tryblidium, Lindstrém.” 
So far he expresses our views exactly, but when it comes to the new species, which 
he names 7’. canadense, we, as did also Lindstrom in his second work on the genus (p. 
54), note differences that necessitate its removal from Tryblidium. To us it is an 
undoubted member of our new genus Archinacella. 
In 1886 Prof. R. P. Whitfield (op. cit.) described two species, ovale and ovatum, 
which are unquestionably congeneric with 7. wnguis Lindstrém. In his remarks on 
the species he points out some differences which exist between his species and the 
generic description given by Lindstrém, 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 7. conicum, in which the apex 
is almost central. Such species should in most cases be regarded as belonging to 
Scenella or Paleacmea. Further, he notes a difference in the number of the 
muscular scars, Lindstrém stating that the Gotland species have only six pairs, 
while his species have eight pairs. Again, he found that his species differed from 
Lindstrém’s generic diagnosis in having the muscular scars “continuous around and 
below the apex of the shell, in a deep and continuous line, from the elongated 
clavate scars on the sides of the beak or apex, as they are in Nacella,” instead of 
“open or nearly so towards the outer end.” 
A careful study of 7. unguis, which we owe to the kindness of Dr. Lindstrém 
himself, enables us to show that there is no essential difference between the muscular 
sears of this Gotland type of the genus and those shown to exist in the Fort 
Cassin beds species by Whitfield. The fact is that T. wnguis has eight instead of 
six pairs of scars, and the narrow ends of the anterior pair, which is smaller than 
