AEGOCERAS LACUNATUM. 331 



(fig. 16) ; they are covered with numerous small ribs, separated by valleys of equal width ; 

 near the umbilical margin there are twenty-five to thirty short, stout ribs, which soon 

 bifurcate into two or three branches, and about the middle of the whorl become strongly 

 flexed towards the aperture, ending in from sixty to seventy costae, thickened towards 

 their termination at the side of the sulcus, which occupies the middle of the siphonal area 

 (figs. 17 and 18). In young specimens the sulcus is sometimes converted into a knotted 

 carina, which, by subsequent evolution, afterwards develops the central sulcus. The test is 

 rarely preserved, and is very thick for so small a shell ; nevertheless, the ornamentation of 

 the shell is sharply preserved on the mould, and all the inequalities of the exterior are 

 repeated in the cast thereof. When the shell is preserved the sulcus appears to be 

 narrower and not so deep. 



Affinilies and Differences. — M. Dumortier,^ who has had many specimens of this 

 species through his hands, says that the ornamentation of Aeg, lacunatum approaches 

 much to those of young individuals of Aeg. Charmassei of the lower zone. Nevertheless, 

 Aeg. lacunatum is much more compressed, its ribs are smaller and much more bent 

 towards the aperture, and the sulcus is narrower and shallower than in Aeg. Charmassei. 

 It forms part of a group of small Ammonites of the zone Ammonites Bavidsoni ; but, 

 whilst the others appear to be limited to this zone, Aeg. lacunatum has a wider distribution 

 in time than either of its other zonal associates. It is rarely found below the zone of 

 Arietites stellaris. It is a very abundant species in many regions of the basin of the 

 Rhone ; the best specimens have been collected from the quarries of Noylay. It forms 

 in France one of the most important Ammonites of the Amal. oxynotus zone. It is rare to 

 find good specimens entire, still the fragments are abundant, and easily recognised. 



Locality and Stratigraphical Position. — The Gloucestershire specimens were collected 

 out of the Lower Lias Shales and Limestone near the Midland Railway-cutting at 

 Lansdown New Road, about the horizon of Amaltheus oxynotus^ and this is the only 

 locality in the county from which I have a record of this Ammonite. It occurs in the 

 Amal. oxynotus zone at Robin Hood's Bay, Yorkshire coast, for the specimen of Arietites 

 Collenotii = denotatus, figured in PI. VI, from the zone Amaltheus oxynotus in Robin 

 Hood's Bay, contains in its body-chamber a quantity of the matrix of the rock from 

 which it was extracted, and in this limestone are several fragments of Aeg. lacunatum, 

 showing both the ribbing on the sides of the whorls and siphonal area, with its deep 

 central sulcus. 



^ 'Depots Jurassiques du Bassin du Rhone,' ii, p. 121. 



