BATHMOPTERUS. A NEW FOSSIL GASTEROPOD GENUS FROM THE SILFRIAN OF ALASKA By Edwix Kirk Of the United States Geological Survey In a rocont paper published in these Proceedings^ I briefly dis-cussed an upper Silurian horizon that is widely distributed in Alaska. In the present paper is described a new gastero[)od genus from this horizon. BATHMOPTERUS, new genus This genus superficially resembles Euomphaloptei-us. This resem-blance is due to the general form of the shell and the possession of a broad marginal flange. Here the resemblance ceases, however. Bathmoptei^s has a Avell defined though nontypical slit band and is apparently referable to the Pleurotomaridae, or possibly the Euomphalidae, rather than the Rhaphistomidae. The genus is Euomphaloid in general appearance. The spire is low and broadly spreading. Six closely appressed whorls are present in the figured specimen. The umbilicus is very large and open. The upper surface of the whorl is convex. Laterally the whorls are flat-tened, giving way again to a gently convex area on the under side of the whorl which marks the zone of contact with the succeeding Avhorl. The inner portion of the whorl is convex and drops off rather abruptly from the contact zone. The growth lines on the upper surface of the whorls are directed backward to the slit band. Below the band the growth lines are sharply flexed backward in the vicinity of the band. The lines are then carried outward on the broad flange or carina, where from a backward attitude they curve gradually until in the marginal area of the flange they are at right angles, or even directed slightly for-ward. The growth lines on the under surface of the flange parallel those of the upper surface. The slit band is no more than a flattened area in which the growth lines abruptly change their direction. So far as seen, the band is not sharply delimited by marginal ridges, nor ' Proceedings United States National Museum No. 2692, vol. 71, art. 20, pp. 1-9, 1927. No. 2763.-PROCEEDINGS U. S. NATIONAL MUSEUM. VOL. 74 ART 18 6641—28
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