>TROPODA. 1026 



1 



l'.nini>hnlns began in nphileta. Typir.illy. this genus is probably restricted to rocks 

 of the age of the Calciferous, and consists of discoidal shells, with the spire concave 

 an<i the base nearly flat. The whorls are narrow, in contact, all exposed, and flat- 

 tened or gently convex beneath, angular at the lower part of the slightly convex or 

 flattened periphery and more sharply angular on the upper side. The anterior 

 extremity of the upper keel marks the bottom of a deep > -shaped apertural sinus 

 or notch. The keel itself is homologous with the slit-band of the I'lfurotomariida; 

 though <>f more simple structure. The outer lip extends forward a trifle farther or 

 about ad far as the innermost part of the upper, while the lower on the whole is 

 nearly direct though slightly insinuated in the outer half. 



From the typical forms of the genus like O. complanata we pass by rather easy 

 gradations through forms like 0. ? bella Billings and Euomphalus unianyulatus Hall, 

 to Helicotoma, Salter, the change consisting in the raising of the spire till it projected 

 -lightly above the level of the last volution, in the deepening of the umbilicus and in 

 the rounding of the outer and lower sides of the whorls. The course of the lines of 

 growth and the > -shaped notch at the end of the upper carina remain about the 

 same. Hut it is not from fully developed Hflicotoma, which continued as an inde- 

 pendent genus to the close of the Lower Silurian, that the later Euomphali were 

 evolved. In our opinion they were derived from a second branch of the Calciferous 

 uniangulatus section of Ophiletn, the first resulting in Helicototna, the farther develop- 

 ment of which tended toward the early pleurotomarian rather than the euomphaloid 

 type of structure. In Helicoioma, namely, the upper carina becomes very much like 

 a slit-band, and that is precisely the opposite of what was necessary to produce an 

 mphalus, in which the apertural notch is reduced to the minimum. 

 Now, if the student will compare species like 0. bella Billings, 0. (En.) uniangu- 

 latus Hall, Eu. obtusangulus Lindstrom, /.'. precursor Lindstrom and K. utalmstedi 

 Lindstrom, the last two Upper Silurian species, we think he will be prepared to 

 admit the correctness of our views, since with our present knowledge it is quite out 

 of the question to arrange the last of the species mentioned in any other position 

 than in the immediate vicinity of the original Carboniferous types of Euomphalus. 



(luring Devonian times a side branch from Euomphalus, for the best species of 

 which Hull proposed the generic name Pleuronotus, became very abundant in 

 individuals if not in species also. It is interesting to note that Plturonotus represents 

 a very striking return to characters pertaining to Oj>hil<t<i. there being the same flat 

 base, carinate upper side, and deep apertural notrh which previously had been the 

 main characteristics of that primitive genus. It might be contended that Ophileta 

 enjoyed a continuous existence from the Calciferous to the Devonian and that 



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