828 THE PALEONTOLOGY OF MINNESOTA. 



[Archill acella. 



apex are a peculiar feature, though probably of the nature of "progressive tracts." 

 The obtuse angulation of the back of the shell distinguishes the species from 

 all the Paleozoic forms of this family known to us. When not in a good state of 

 preservation it might be confounded with Archinacella patelliformis Hall sp., but in 

 that species the angulation of the back does not extend to the extremities of the 

 shell, while the anterior end is wider, the apex higher and the surface without 

 radial markings. 



Formation and locality. Trenton group, Clitambonites bed, Goodhue county, Minnesota. 

 Colkction. E. O. Ulrica. 



Genus ARCHINACELLA, n. gen. 



Metoptoma (part.), BILLINGS, 1865, Palaeozoic Fossils, vol. i. WHITFIELD, 1878, Geol. Wis., vol. iv. 

 Tryblidium (part.), WOITEAVES, 1884, Paleozoic Fossils, vol. iii, p. 31. WIIITFIELD, 1886 and 1889, 

 Bull. Amer. Mus. Nat. Hist., vols. i and ii. 



For generic diagnosis see page 821.* 



The shells which we propose to refer to this genus are decidedly like those of 

 Tryblidium, especially so far as the position of the apex and the surface markings 

 are concerned. Their internal markings, however, are readily distinguished, that 

 genus having the muscular scars in eight detached pairs, while they form a 

 continuous band in Archinacella. Unfortunately the muscular imprints are in most 

 cases very faint, even on well preserved casts,' so that we are generally obliged to 

 rely upon another character in determining the generic position. Namely, in all 

 the species of which the muscular scars are known to occur in detached pairs the 

 anterior outline is acuminate, or at any rate more narrowly rounded than the 

 posterior margin. On the other hand, the anterior margin is as broadly rounded or 

 wider than the posterior outline in all the forms of which it is known that their 

 muscular scars are not detached. We have, therefore, considered it good practice 

 to assume that when the anterior end is narrowly rounded the species is a Tryblidium 

 and when this end is the wider the species belongs to Archinacella. 



There may be some doubt about the affinities of that group of shells in which 

 the outline, as viewed from above, is almost circular or regularily elliptical. In no 

 case have we been able to make out the muscular scars, although we have studied 

 some well preserved casts. Still, as the form of these shells agrees best with 

 Archinacella, and as we know nothing seriously opposing our view, we think it best 

 to arrange them, at least provisionally, under this genus. Besides the species of 



We omitted from the generic diagnosis one feature that ought perhaps to have been Included, namely, a pair of scars 

 i'- muscular) occurring one on each side of the apex. They lie on tin- outside' of the usual muscular band and have been 

 observed In two species, A. poutent and A. (Tryblidium) canadensi* Whiteaves. The latter Is a CJuclph species and, as shown 

 In Whlteaves' figures n'al. FOBS., vol. Ill, pi. v), haa these scars more strongly impressed (in the cast) and further forward 

 than they are ID A. powertl. 



