DORSETENSIA SUBTECTA. 
309 
to retrogressive species of other genera ; as, for instance, Hyperlioceras, Haugia, 
and Htidlestonia — from any of which, however, the suture-line alone effectually 
distinguishes this group. 
DoESETENSiA SUBTECTA, S. BucJcman. PI. LIV, figs. 3 — 5 ; PI. LV, figs. 1, 2. 
1886. Ammonites TESSONiiNUS, Quenstedt (non d'Orhigny). Schwabischen 
Ammoniten, pi. Ixiii, fig. 8. 
Discoidal, compressed, hollo w-carinate. Whorls broad, much compressed, 
ornamented with arcuate, ventrally-proj acted ribs declining to striae in the 
adult. Ventral area not defined, furnished with a very strong hollow carina. 
Inner margin well-defined, upright. Inclusion about one-half. Umbilicus 
graduated, marked with obscure ribs. Suture-line with broad-stemmed lobes 
and shallow saddles ; two accessory lobes in the siphonal saddle. 
In this species I have combined three varieties which differ from one another 
in certain features : 
a. (PI. LIV, fig. 3.) Umbilicus rather open and ribbed, obscure ribs on the 
whorl retained for some time. 
j3. (PI. LIV, fig. 4.) Umbilicus smaller, and less conspicuously ribbed ; the 
ribs on the whorl yielding to striae at an earlier age. 
y. Like j3, but thinner, whorls flatter, umbilicus smaller, and without any trace 
of ribs. 
The form a, which is the earliest biologically, connects this species with 
complanata " — in fact, the hollow carina is the only feature which distinguishes 
it therefrom. 
The form /3 has very considerable resemblance to Am. Tessonianus, d'Orbigny ; 
but, in addition to other minor points of difference, it possesses a far larger and 
more distinctly-raised carina, which effectually separates it from d'Orbigny's 
species. 
The form y, being thinner and flatter than (3, must be looked upon as a side- 
shoot ; because increasing thickness seems to be a feature in the direct line of the 
" Liosiraca-group." The decrease in the size of the umbilicus in the three speci- 
mens figured in PI. LIV, and in the form y, is very gradual from one to the other, 
but is a noticeable feature. The umbilicus of " subtecta y " \q one-third less in 
diameter than that of the " comjplanata " figured in PI. LIV. I prefer to con- 
sider the form j3 as the type of this species. 
Dors, suhtecta occurs in the Humjjhriesianum-zone of Oborne and Sherborne, 
Dorset, but is decidedly scarce. A good example of the form is shown in fig. 3, 
