imACHlOPODA. 359 



SiphoiiotretaV minneaoteDsls,] 



convexity toward the lateral and anterior margins. Foramen apical (?). Brachial 

 valve depressed-convex, somewhat elevated about the umbo. Surface covered, 

 in the umbonal region, with fine, anastomosing and gently undulating concentric 

 lines, which, in the latter portions of the shell, are finely granulose or serrated; at 

 about one-third the length coarser varieties of growth appear, between which the 

 finer lines are retained. Surface covered with hollow spines of various sizes, which 

 appear to have been mostly set over the umbonal region of the pedicle valve. 

 Here, where the growth lines are absent, the spine-bases in the original specimen are 

 large and all of about the same size, and are disposed without order. Over the other 

 portions of the shell the spines are set along the edges of the varices, small and large 

 being indifferently mixed. The bases of the spines make annular swellings on the 

 interior of the valve. The length of the original specimen is 15 mm., width, 12 mm." 

 Since no additional material has been discovered other than that used by 

 professor Hall, we give in addition his observations on the species, with its 

 relations to a similar form which is tentatively referred by him to Schizambon. 

 "The American palaeozoic faunas have yet furnished no thoroughly satisfactory 

 representative of Siphonotreta. Before us are two specimens of a form allied to, but 

 probably distinct from the Canadian representatives of Schizambon (?), referred to 

 on a following page [Siphonotreta scotica White&ves=Schizambon fissus var. canadensis 

 Ami (Hall)], 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 part of the 

 pedicle valve has been broken, leaving no indication of the character of its foram- 

 inal aperture. The shell differs somewhat from the Canadian specimens in outline, 

 being broader over the pallial region; the brachial valve shows a low longitudinal 

 depression, the shell substance 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 Siphonotreta, with the designation 

 Siphonotreta minnesotensis.' 1 



Formation and locality. From the Trenton limestone near the University of Minnesota, Minne- 

 apolis, Minnesota. 



Collectors. E. O. Ulrich and Charles Schuchert. 



