DESCRIPTIONS OF NEW SPECIES OF FOSSILS. 



SILUPtlAN FORMS. 

 Iphidea (??) sculiMUs, Meek. 



This fossil presents very much tbe general appearance of the ventral 

 valve of an Acrotreta, being rounded on one side and truncated on the 

 other, with the apex moderately prominent, and marginal on the trun- 

 cated or posterior side, (viewing it as an Acrotreta,) and with the trun- 

 cated side inclined backward. It measures 0.11 inch in breadth, and 

 about the same in length, with a direct height to the apex of 0.05 inch. 

 Its surface has a black shining appearance, indicating a phosphatic 

 composition like Lingula, and is marked by slender, interrupted, rather 

 distant, radiating raised lines, crossed by finer, much more crowded, 

 and very regular, sharply-defined, concentric strife. So far as can be 

 seen, there seems to be no perforation in the apex. 



On first looking at these little shells, which occur associated with a well- 

 defined Acrotreta, I had not the slightest doubt that they belonged to a 

 depressed species of that genus. By cutting away the hard rock, how- 

 ever, from the flattened side, corresponding to the area of Acrotr eta, I 

 found that there seems to be there a wide, open, triangular foramen, 

 so large that only a very narrow, slightly flattened margin, represent- 

 ing a false area, is seen inflected on each side. This I have seen in the 

 only two specimens in the collection showing this side ; and if there is 

 nothing deceptive about it, the shell would certainly belong neither to 

 Acrotreta nor to Iphidea. It can only be referred to Mr. Billings's genus 

 Iphidea, even provisionally, on the supposition that, in cutting away the 

 hard rock from the truncated side representing the false area in that 

 genr.s, I may have also cut away the prominent pseudo-deltidium, char- 

 acteristic of that group. In that case, however, the pseudo-deltidium 

 would be proportionally much wider than in Mr. Billiugs's type, as the 

 opening is decidedly wider, as we see it in our shells, than the false del- 

 tidium in his species. This difference, however, might be only sijecific. 



It is quite probable that, when specimens showing clearly all the 

 characters of this shell can be examined, it will be found to belong to 

 an undefined genus, either of the Brachiopoda, or of some other group. 

 In this case I would propose for this genus the name Micromitra. 



I confess, however, that in closely examining these little fossils, I have 

 not been entirely without the suspicion that they may be the terminal 

 pieces of some extinct group of the Chitonidcc. The inflected character, 

 however, of the margins, like a very contracted false area, on each side 

 of the opening of the flat side of the shell, is against this conclusion; 

 but even if they are the terminal pieces of some chitonoid type, the 

 chances are still strongly in favor of its being a new genus, for which 

 the name suggested would be equally as appropriate as for a Brachiopod. 



Locality and ppnition. — East side of Gallatin River, Montana; pri- 

 mordial zone. 



