952 



THE PALEONTOLOGY OF MINNESOTA. 



[Synopsis of PleurotomaritdaB. 



II. SCHIZOLOPHA, n. gen. (Ulrich.) In every respect like Lophospira excepting 

 _ that the apertural notch is prolonged into a long parallel-edged slit. Type, S. 



textilis, n. sp. (Ulrich.) 



III. PHANEROTREMA, Fischer, 1885.* Shell turbinate, whorls few in number, 

 flattened above, the last large and high, the others rising step-like; spire short; slit 

 long, region of band salient; aperture subquadrate, inner lip thick; lines of growth 

 nearly vertical beneath the band, above it turning more decidedly forward; strong 

 revolving lines occur especially on the lower and outer parts of the whorls. Type, 

 P. labrosa Hall. 



This Upper Silurian genus seems to have been derived from the Trochonemoides 

 section of Lophospira, our L. trochonemoides and L. knoxvillensis being very similar in 

 general form and having almost exactly the same kind of aperture. The band, 

 though salient also in those species, is somewhat different, the central line being, as 

 it should be in Lophospira, considerably stronger and more prominent than the 

 bordering lines. Still, in our opinion, the peculiarities of the labrosa type of band 

 were produced by an extreme development of the bordering lines, the space between 

 the elevated edges being gently convex. Aside from the band, Phanerotrema is 

 distinguished from all the Lower Silurian Pleurotomariidce, except Schizolopha, by its 

 long slit, and from the excepted genus by its relatively smaller spire, much larger 

 last volution, peculiar band, and strong revolving lines. Further, it seems very 

 clear that, although both types originated in Lophospira and deviated from the 

 general character of that type in similar directions, their immediate ancestors 

 represent widely different sections of that genus, and constitute distinct lines of 

 development. Of the two species of Schizolopha, S. textilis evidently came from the 

 Bicinda section, while S. mooresi accords more nearly with the Perangulata section. 

 As to Phanerotrema, we have already said that it was most probably derived from 

 the Trochonemoides section. 



Phanerotrema includes besides the type species, PI. occidens Hall, and a Goth- 

 landic shell which Lindstrom erroneously identified with Hall's labrosa. The genus 

 appears to be sparingly represented in the Devonian of Europe, but we know of 

 none in American deposits of that age. Two of our Carboniferous species, however, 

 PI. grayvillensis Norwood and Pratten, and PL marcouiana Geinitz, appear to have all 

 the essential characters of Phanerotrema, and we expect to find that they are actual 

 continuations of the same generic type. 



IV. WORTHENIA, De Koninck, 1883. Shell conical, tabulate, the general aspect 

 much as in Lophospira; whorls angular on the periphery, the latter carrying the 

 band; slit extending backward from the mouth between one-fourth and one-third of 



Muuual lie Concbyliologie. p. 651. 



