'.M'.t 



1 



In <uif group <>f I'/, nrtniHitrii<l<i. i. e. //"' \ve havegood evidence showing 



a gradual development of the -lit. In all the Lower and Upper Silurian species of 

 thi- genu-a deep V-shaped ape rtural notch i- ptv-i-nt. hut no -lit. I n, however, what 

 we con-i'ler to be Devonian repn . es of the same type of shell (e. g. ^furchi- 



sonin "itn and main Hall) we observe that the bottom of the notch is prolonged 



into a short -lit. luit the I ark ward sweep of the edges of the outer lip forming the 

 notch is unite a- pronounced as in the earlier species which have no slit. From 

 tin- and the preceding case, therefore, it is evident that the slit did not take the 

 pl.ire of a deep notch but that it is really an additional and distinct feature. 



The length of the >lit varies greatly in different members of the family. On the 

 whole the length is considerably greater in Mezozoic than in Paleozoic species. In 

 the latter the length has not been observed to exceed three-eighths of the circum- 

 ference of the last whorl. In the Devonian PI. sul<-om<ir<iinuta it is about one-fourth, 

 likewise in the /'/. tnrlrini/nrmi.v group i i'.uconospira ), the PI. tnlmlnta or Worllienin 



.p and the majority of the Carboniferous species. In Schizolopha it is a trifle 

 longer, while in the Niagara /'/. lubrosa group (Phanerotrenut Fischer) it equals about 

 one-third. In PI. njihurulntn it is about one-seventh, and in Miin-liinonia maia Hall 

 not over an eighth, while in the PI. carbonaria group it is even shorter and possibly 

 absent entirely. Among Mesozoic and more recent forms, particularly Lei>t<nari(i 

 and Chelolin. the length often exceeds one-half and may reach fully two-thirds 

 <>f the last whorl. It is interesting to note that, as far as we now know, the 

 slit which furthermore seems to have been developed almost suddenly, is longer 

 in the earliest species known to possess one than in any of the later Paleozoic 

 forms. 



Th- land which i- left behind by the gradual closing of the slit presents 

 considerable variety in position and structure. As a rale, especially among 

 Paleozoic species, it lies on the peripheral part of the whorls. When the volution- 

 are angular it commonly forms the summit of the principal angle, a- in Lnphospira 

 and \\'<>rthenia. In conical shells, like those of the group of PI. turbhil/nniii*. it forms 

 a narrow vertical band at the extreme periphery of the whorls. In other conical 

 shells, like / Killing, it lies at the base of the flat slope, the lower edge of the 



band in -m-h ca-es funning the periphery. In certain Mesozoic ami recent types 

 (Pyr :' .UP! /..,'. -..I '/.<',/- (.; Fitther) in which the shell is 



similarly conical, the b.ind lies considerably above the angular periphery, but it 

 never occupies such a position in any of the angulati-1 1'aleozoic t\pe-. l.eing in 

 these placed al\\.r. - very near or entirely upon the angle. Occasionally, as in the 



oniferous group of PI. Lrnzoensis, it lies in a broad peripheral concavity, beneath 

 the principal carina. 



