DORSETENSIA TECTA. 
311 
If fig. 13 and figs. 8 and 9 of this plate be considered together, a very fair 
idea of the ontogeny either of " complanata,^' or of a member of the " Liostraca- 
group " may be gained. 
In fig. 15 of the same plate is shown a young specimen ia which no hollow 
carina is developed. In fig. 11 is given an older specimen, which has a distinct 
hollow carina developed only on the end of its last whorl. In fig. 4, PI. LV, is 
shown a still more advanced specimen, which bears a hollow carina all round ; 
while the side-view of an adult example is depicted in fig. 3.^ Two suture-Hnes of 
immature examples are depicted in figs. 12 and 16 of PI. LIII, and an outline 
section of a young specimen in fig. 14 of the same plate. 
The variety in which ribs are shown in place of the striee is illustrated by two 
examples, PI. LV, fig. 5 ; PI. LVI, fig. 1. 
DOKSETE NSiA TECTA, 8, T3uchiuciu. PI. LVI, figs. 2 — 5. 
The general description of this species agrees with that of Dors, liostraca, but 
the following points of difference may be noted : — The inner margin is deeper 
and is distinctly convex, the umbilicus is (before the sudden expaasion) decidedly 
smaller as well as deeper, the whorls are thicker, and the aperture is conse- 
quently broader and more obtusely-lanceolate. 
More or less pronounced indications of these features may be seen in the larger 
examples of Dors, liostraca towards the end of the last whorl, so that the 
differences between this species and the last may be summed up by saying 
" the senile features of ' liostraca ' have become adolescent and adult features of 
' tecta: " 
The above statement must be qualified in one particular, namely, in regard to 
the umbilicus. The tendency to contract the umbilicus is observable throughout 
the whole of the " Liostraca-group ;" but it is followed by a tendency to 
subsequent expansion of the umbilicus. Both tendencies are inherited earlier and 
earher in the series, but owing to the fact that the contracting tendency 
precedes the expanding tendency, the latter, has, in the later species, a less 
influence. 
In the forms of " suhtecta " there is a notable contraction in the umbilicus ; 
but the subsequent expansion is not very marked. In "liostraca" the contrac- 
tion is still more pronounced, and the subsequent expansion becomes a well- 
marked feature. In " tecta " the increased contraction causes the whorls to be 
^ In front view this specimen is just between " subtecta'' and ''tecta" — rather thicker than the 
former, and rather thinner than the latter. 
