| Measurements | D mm | H/D | T/D | O/D | H/T |
|---|---|---|---|---|---|
| Rodolphe Touch | 63 | 0.46 | 0.40 | 0.21 | 1.16 |
| CP-158 | 69 | 0.45 | 0.42 | 0.22 | 1.07 |
| Holotype Collignon | 86 | 0.45 | 0.40 | 0.26 | 1.15 |
| CP-142 | 89 | 0.47 | 0.37 | 0.20 | 1.27 |
| M. falcistriatus Anthula | 92 | 0.48 | 0.44 | 0.25 | 1.03 |
| Age | Origin |
|---|---|
|
Lower Albian D. inaequinodum Malagasy zone |
Befamonto Boeny Region Madagascar |
Description. Involute ammonite with a nacreous test and 110° of body chamber. The whorls are fairly thick and ovoid, 70% covered, with maximum thickness near umbilicus and a broad, convex venter. The umbilicus clearly shows the inner whorls, with a subvertical wall but a broadly rounded margin. The internal mold bears six deep constrictions. These originate obliquely on the umbilicus wall, straighten after the umbilical margin, then follow a radial segment up to mid-flank and finally a large concave arc, giving them an overall falcate appearance. These constrictions cross the venter with a strong proverse sinus, almost angular at 90° initially, then wider and rounded on the body chamber. They have a anterior edge steeper than the posterior and narrow, or even close up, on the siphonal line. Apart from these constrictions, the internal mold is smooth. On the test, the constrictions are hidden or barely visible, but from 3-4 cm in diameter they are bordered posteriorly by a fine, low rib, distinct only on the outer half of the flanks and becoming more pronounced during growth. Between these ribs, the test is covered with very fine, closely spaced, falciform growth lines.
Remarks. We have examined four specimens, with fairly variable O/D and H/E ratios, but all conforming well to the Malagasy variant mitsinjensis of Collignon (1963). They share the same proportions as Anthula's M. falcistriatus (see the genus entry), a small umbilicus with a vertical wall, sinuous constrictions, and a long, slender, trifid first lateral lobe. However, the variant has a slightly more compressed whorl section, a less broad venter (more "triangular" according to Collignon), a stronger ventral sinus, and no intermediate ribs. This species is often confused with Desmoceras latidorsatum on the Internet. The latter (see its entry) may lack any constrictions; it has a smaller umbilicus with a more narrowly rounded edge, and parallel flanks that give it an even broader venter.