GASTROPODA. 949 
Pleurotomariid®.] 
In one group of Pleurotomariide, i. e. Hormotoma, we have good evidence showing 
a gradual development of the slit. In all the Lower and Upper Silurian species of 
this genus a deep V-shaped apertural notch is present, but no slit. In, however, what 
we consider to be Devonian representatives of the same type of shell (e. g. Murchi- 
sonia desiderata and maia Hall) we observe that the bottom of the notch is prolonged 
into a short slit, but the backward sweep of the edges of the outer lip forming the 
notch is quite as pronounced as in the earlier species which have no slit. From 
this and the preceding case, therefore, it is evident that the slit did not take the 
place of a deep notch but that it is really an additional and distinct feature. 
The length of the slit 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 nos been observed to exceed three-eighths of the circum- 
ference of the last whorl. In the Devonian Pl. sulcomarginata it is about one-fourth, 
likewise in the Pl. turbiniformis group (Euconospira), the Pl. tabulata or Worthenia 
group and the majority of the Carboniferous species. In Schizolopha it is a trifle 
longer, while in the Niagara PI. labrosa group (Phanerotrema Fischer) it equals about 
one-third. In Pl. spherulata it is about one-seventh, and in Murchisonia maia Hall 
not over an eighth, while in the Pl, carbonaria group it is even shorter and possibly 
absent entirely. Among Mesozoic and more recent forms, particularly Leptomaria 
and Chelotia, the length often exceeds one-half and may reach fully two-thirds 
of 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. 
The band which is left behind by the gradual closing of the slit presents 
considerable variety in position and structure. As a rule, especially among 
Paleozoic species, it lies on the peripheral part of the whorls. When the volutions 
are angular it commonly forms the summit of the principal angle, as in Lophospira 
and Worthenia. In conical shells, like those of the group of Pl. turbiniformis, it forms 
a narrow vertical band at the extreme periphery of the whorls. In other conical 
shells, like PJ. etna Billings, it lies at the base of the flat slope, the lower edge of the 
band in such cases forming the periphery. In certain Mesozoic and recent types 
(Pyrgotrochus, Perotrochus and Entemnotrochus of Fischer) in which the shell is 
similarly conical, the band lies considerably above the angular periphery, but it 
never occupies such a position in any of the angulated Paleozoic types, being in 
these placed always very near or entirely upon the angle. Occasionally, as in the 
Carboniferous group of Pl. brazoensis, it lies in a broad peripheral concavity, beneath 
the principal carina. 
