DORSETENSIA COMPLANATA. 
307 
thin, and very flat. They are ornamented only with obscure, undulate,^ sub- 
arcuate, ventrally-projected ribs, which become more and more obscure with age. 
Ventral area not defined, sharp, furnished with a small carina which becomes 
almost obsolete with age. Inner margin ill-defined, sloping. Inclusion one-half, 
decreasing to one-third. Mouth-border a sub-arcuate bend with a bluntly-pointed 
ventral process. Suture-line simple ; siphonal saddle with two accessory lobes ; 
superior lateral lobe broad-stemmed. 
This species has a certain resemblance to Dors. Bomani (Oppel) but a greater 
likeness to Dors, ptdchra ; and in its style of ribbing it resembles the outer whorl 
of " Edouardiana S." Still I am not altogether satisfied that it is the descendant of 
Edouai'diana and it is certainly not the descendant of " piilchra.^^ It is true 
that the ribbing of this species is just what the descendant of "Edouardiana " should 
possess, as is shown by the last whorl of " Edouardiana S " ; and the general shape is 
what might be expected. But the one point which rouses my suspicions is that the 
superior lateral lobe of the immature shell is longer than the siphonal ; whereas it 
should be, like that of adult "Edouardiana,'" of the same length. This may seem 
a little matter ; but it is really not to be overlooked. It may mean that " coni- 
planata " and its allies belong to another branch, and should not be included in 
Do7'setensia. 
The undulatory character of its ribs, the increase in the size of the umbilicus 
during the last whorl, and the sub-carinate ventral area are the features which 
separate Dors, complanata from any other species of the genus. 
Dors, complanata is a scarce fossil from the Htimphriesianum-zone of Sherborne 
and Oborne, Dorset. The specimen which my father attributed to Bradford Abbas 
is depicted in fig. 1 ; its matrix shows that this was an error. 
Fig. 1 of PI. LIII gives the side view of an example with the mouth-border ; 
fig 2 is a view from the top showing the somewhat inflated mouth with the ventral 
lappet, and that the specimen is thicker than its fellows. Figs. 8, 4 mark two 
views of the thin form, which has on its body-chamber two long spatulate 
depressions — a character seen on other specimens of this species, but not so 
plainly ; fig. 5 is the suture-line. Fig. 6 is the front view of an example with very 
compressed sides and acute ventral area. Fig. 7 is the suture of another example 
showing inner lobes dependent — an abnormal feature for this species. Figs. 8, 9, 
10 illustrate the ontogeny of the species, showing the change of whorl-shape, and 
the gradual acquirement of ribs. Fig. 1, PL LIV, marks a somewhat stronger- 
ribbed, slightly more involute example of the species ; fig. 2 is its suture-line 
taken from about the commencement of the last whorl. This specimen may be 
regarded as slightly earlier, biologically, than the others. 
^ Ribs which are broad, rather distant, and not very well-marked, in distinction to the sharply - 
cut ribs of the previous species of the genus. 
