Desmoceras (D.) latidorsatum  (Michelin, 1838)  var. medium  Jacob, 1907

Measurements	D mm	H/D	T/D	O/D	H/T
CP-591	70.4	0.45	0.38	0.24	1.18
CP-321	146.4	0.47	0.41	--	1.14

Age	Origin
Lower Albian Malagasy zone of Lemuroceras spathi & Brancoceras besairiei	Ampanihy Boeny Region Madagascar

Description. Compare with the entry for D. latidorsatum sensu stricto. A large specimen of 14.6 cm with the beginning of its body chamber on the last quarter of whorl. The section is oval with a broad venter, a bit crushed near the aperture, hence the somewhat convergent flanks. In fact, the lower flanks are parallel at the beginning of the last whorl. The deep umbilicus with a vertical wall shows the three previous whorls. Its wall joins the flanks perpendicularly with a narrowly rounded edge. On the test preserved on the other side, 10 low, thin sigmoid ribs cross the venter with a forward rounded sinus. They are barely visible on the inner half of the flanks. The side without test of the picture shows a large serpula and the constrictions hidden under the ribs on the internal mold. Shallow and originating on the umbilical margin, they are wide to mid-flank, with an anterior edge steeper than the posterior, then they narrow towards venter. Between the ribs, very fine sigmoid growth lines cover the test.

Remarks. With their H/T ratios between 1.10 and 1.20, our two moderately compressed specimens correspond to the variant medium Jacob, 1907 of Desmoceras latidorsatum. According to Collignon (1949), this is the second most frequent variant in Madagascar, representing 27% of specimens, after Desmoceras latidorsatum sensu stricto (64%). The most compressed form is the complanatum variant, also defined by Jacob (1907), with an H/T ratio greater than 1.20. It represents 8% of Malagasy specimens.