312 
INFERIOR OOLITE AMMONITES. 
almost occluded^ just before the commencement of the expansion — in fact, "85 of 
the whorl is covered ; but with the expansion of the umbilicus this is decreased 
to "57 in one turn of the whorl.^ 
Dorsetensia tecta occurs in the Humphriesianum-zone of Oborne, Dorset, but 
is rare. 
Figs. 2, 3, 4, of PI. LVI, exhibit side and front views and suture-line of a 
wholly septate specimen. The test is mostly preserved, but the hollow carina is 
lost. Fig. 5 of "the same plate shows a younger example with a more included 
umbilicus ; its carina has also been broken away. 
The annexed Table summarises the remarks on the afl&nities of the various 
forms of Dorsetensia. 
Table VII. — The Genealogy of Dorsetensia. 
Dors, tecta 
. I Dors, liostraca, y. 
Dors, liostraca, /8 
Dors, liostraca, a 
Dors, subtecta, /3 
I 
Dors, subtecta, o 
-Dors, subtecta, y. 
-Dors, complanata. 
Dors. Edouardiana, S 
Dors. Edouardiana, y 
Dors. Edouardiana, ^ 
Dors. Edouardiana, a 
Dorsetensia sjj. A 
-Dors, pulchra — Dors. Eomani. 
-Dors. Edouardiana, a a. 
-Dors. sp. B. 
1 Hence the name " tecta, covered." 
- It must be noticed that in the series of young and old of this and other groups a certain 
expansion of the umbilicus is to be seen whenever the body-chamber is preserved. If this 
expansion were maintained in subsequent growth the umbilicus of adult specimens ought to be alway? 
larger than is found to be the case ; but, presumably, as the air-chambers were formed the inner edge 
of the whorl was pushed nearer to the centre. With failing vital powers either in the individual or 
the race, there might not be sufl&cient strength to carry out this process. A permanently larger 
umbilicus would then result, and this character would be transmitted earlier and earlier to the 
dfcscendants. 
Hyatt, referring to the same phenomenon in regard to the umbilicus of StepJi. plafi/sfomum,'Rem. 
(hullatum, d'Orb.), supposes "that the living chamber must be wholly absorbed in the course ot 
growth " (" G-enetic Eelations of Stephanoceras,'" ' Proc. Boston Soc.,' vol. xviii, p. 397 [37], 1S76). 
