112 PALEONTOLOGY OF NEW YORK. 



in the absence of an accurate understanding of the internal impressions, there' 

 appears to be no feature of generic value by which they may be separated ; it 

 must, however, be admitted that the resemblance in general form and propor- 

 tions of these latter shells to the true Siphonotbeta is very close. 



The American palaeozoic faunas have yet furnished no thoroughly satisfac- 

 tory representative of Siphonotreta. Before us are two specimens of a form 

 allied to, but probably distinct from the Canadian representatives of Schizam- 

 BON (?), referred to on a following page, which have been collected by Mr. 

 Charles Schuchert and Mr. E. 0. Ulrich, from the Trenton limestone at 

 Minneapolis, Minnesota. One of these is an exterior mould, the other retains 

 both valves, though the umbonal portion of the pedicle-valve has been broken, 

 leaving no indication of the character of its foraminal aperture. The shell differs 

 somewhat from the Canadian specimens in outline, being broader over the pallial 

 region; the bi'achial valve shows a low longitudinal depression, the shell-sub- 

 stance is very thin, while in the other species referred to it is remarkably thick, 

 and its lamellose structure conspicuously developed ; the ornamentation of the 

 surface consists, not of sharp, concentric lines, broadening to ridges toward the 

 margin, but of fine, concentric, anastomosing wrinkles, which are interrupted 

 over the body of the shell by the edges of the spiniferous lamellae ; the spines 

 are comparatively short and sparse. Though recognizing the difficulties in the 

 discrimination of species of Siphonotreta, we are nevertheless disposed to 

 regard the above-mentioned features as of specific value ; and in the absence 

 of evidence determining the character and position of the pedicle-passage, it 

 seems judicious to leave the species for the present under the genus Siphono- 

 treta, with the designation, Siphonotreta Minnesotensis. 



The internal features of this genus are not so precisely known as to permit 

 the satisfactory determination of homologies with other genera. Dr. David- 

 son's figures of the interiors of both valves of S. unguiculata, published in the 

 Geological Magazine for 1877, and reproduced upon our Plate IV, differ in some 

 important particulars from the earlier figures given by Kutorga, essentially in 

 the absence of the nipple-like swelling in the umbonal region of the pedicle- 



