426 G. F. Matthew — On the Genus Stenotheca. 



the Heteropods and Grasteropods ; but the order to which these fossils 

 should be referred is still doubtful, and must await further knowledge. 

 The little shells of this genus found at St, John are always inequi- 

 lateral, and usually present a carina at the posterior (?) end, and not 

 unfrequently at both ends. Among recent Molluscs, Garinaria and 

 Atlanta are the forms which appear to come nearest to the species 

 under consideration. In general outline the fossils are nearest the 

 former genus, but in their compressed forms they remind one of the 

 latter ; from both, however, they differ in the straight, or nearly 

 straight apex, and in the arrangement of the lines of growth on the 

 shell. They resemble these genera in that the majority of the 

 species have a keel along the back, and it seems probable also that 

 in some of the species the shell consisted of two latei'al plates 

 imperfectly connected along the back, for the two sides are 

 occasionally found detached, and one pushed past the other. The 

 apex of two out of the five species in the St.-John Group is 

 known, and does not show any tendency to a coiled form, as in 

 Carinaria, Atlanta, or any other Heteropods, with which these fossils 

 may be compared. Among certain genera of Gasteropods, however, 

 as Dentalium, Farmophorus, and Patella, similar apices may be 

 observed. These Stenotheca appear to have been denizens of the 

 open sea, and have only been found in the fine dark shales of 

 Division Id, or about the horizon of the Menevian Group of Wales. 



In the measures of Division Ic containing a fauna, the forms 

 of which resemble those of the Solva Group, is a species of Mollusc, 

 which, although agreeing with the typical Stenothecce of Division Id, 

 shows important points of difference. This species, originally 

 described as Discina Acadica by Prof. C. F. Hartt, exhibits relations 

 with Metoptoma (?) rugosa of the Trenton limestone ; it is much 

 larger than the true Stenothecce, and is preserved in a difierent 

 attitude in the shales. The Stenotheca are flattened on the vertical 

 plane of the shell ; but this species is found compressed horizontally, 

 or parallel to the aperture ; nevertheless, in their younger stages of 

 growth these larger Stenothecce show a tendency to collapse laterally, 

 in this resembling the small shells of the overlying measures ; in 

 general form, also, as well as in the way in which additional con- 

 centric ridges are inserted at the back of the shell, these conform to 

 their congeners in the horizon above. 



Except for the absence of an involute apex, these shells are not 

 unlike Carinariopsis carinata, Hall, of the Trenton formation ; they 

 are compressed near the apex in a manner similar to that species, 

 and expand rapidly toward the aperture ; but the arrangement of the 

 lines of growth or surface-markings is quite different, and comparable 

 with that of Metoptoma (?) rugosa, Hall. Our shells, however, are 

 not Metoptomce, since they are not truncated at the side beneath the 

 apex, as are those of the Carboniferous genus Metoptoma. These 

 patelloid Stenothecce lived in shallow seas near the coast-line, if one 

 may judge from the species of other genera associated with them. 



St. John, N.B., 15th June, 1885, 



