Alcidellus is an oxyconic (sharp ventered) haploceratacean

Haplocerataceae

Haplocerataceae is a superfamily of ammonoid cephalopods belonging to the Ammonitida that unites three families, the Strigoceratidae, Oppeliidae, and Haploceratidae, listed below....

 ammonite

Ammonitida

The Ammonitida is an order of more highly evolved ammonoid cephalopods from the Jurassic and Cretaceous time periods, commonly with intricate ammonitic sutures....

 from the Middle Jurassic.

Alcidellus was named as a genus buy Gerd Westermann in 1958. It is now generally seen as a subgenus of Oxycerites

Oxycerites

Oxycerites is an extinct ammonoid cephalopod belonging to the haploceratacean family, Oppeliidae, that lived during the middle of the Jurassic Period, between 164 and 160 million years ago...

, differing from Oxyceratites (O) in having a broader venter with ventrolateral shoulders. The shell of Oxycerites, including O (Alcidellus) is generally smooth, essentially involute with a small umbilicus, highly compressed with flanks converging on a narrow, somewhat sharp venter. Sutures are ammonitic.

References

• An Early Bathonian Tethyan Ammonite Fauna from Argentina, by Alberto C Riccardi and Gerd E.G. Westermann. Palaeontology, Volume 42. no2, pub online 21 Nov 2003.

http://onlinelibrary.wiley.com/doi/10.1111/1475-4983.00069/pdf

• Upper Bathonian and lower Callovian ammonites from Chacay Melehue

(Argentina), by Horacio Parent. Acta Palaeontologica Polonica 43, 1, 69-130. 1998. http://www.app.pan.pl/archive/published/app43/app43-069.pdf