AEGOCERAS SUBMUTICUM. 339 



Diagnosis. — Shell large, flattened on the sides, and slightly involute, umbilicus 

 widely open ; spire formed of eight whorls which have a convex margin and are slightly 

 compressed on the sides, the last whorl has thirty-eight slender principal ribs, which are 

 only slight elevations, and terminate in sharp prominent tubercles near the outer border of 

 the siphonal area ; the ribs are very feebly marked on the inner volutions ; on the last 

 whorl they first bend backwards, then incline forwards, and again bend back and terminate 

 in tubercles. The siphonal area is broad and slightly convex ; from the lateral tubercles 

 transverse folds extend across the area, with several smaller elevations between ; aperture 

 subquadrate, wide and convex above, flattened on the sides, contracted and grooved below 

 by the turn of the spire ; shell extremely thin and well preserved in several parts. 



Dimensions. — Transverse diameter 140 millimetres ; width of the umbilicus 60 milli- 

 metres ; height of the aperture 50 millimetres; greatest width above 50 millimetres; 

 below, over the spire, 85 millimetres. 



Description. — This is a rare form, and closely resembles the Am. natrix oUongus, 

 Quenst., of the ' Cephalopoden,' and is one of the most elegant species of the Armati group. 



The shell attains a considerable size, is gently compressed on the sides, and has a 

 very wide umbilicus ; it is, therefore, extremely evolute and fully exposes all the inner 

 turns of the spire, which is formed of eight volutions that are round and slightly com- 

 pressed and covered with delicate ribs ; the last whorl has thirty-eight principal ribs, 

 which are shghtly elevated and directed forwards, and they all terminate in small tuber- 

 cles near the outer border of the siphonal area. The ribs on the inner whorls are very 

 slight elevations, whilst on the outer whorl they describe with sigmoidal curves, 

 smaller intermediate secondary folds lying between them. The siphonal area is wide, 

 convex, and without a trace of keel ; large transverse folds pass across the area from one 

 tubercle to another, with one or two smaller intermediate bands between the larger folds. 

 The shell of this Ammonite is extremely thin, and very well preserved on some parts 

 of the figured specimen, and shows that the tubercles were hollow cones filled in with 

 the matrix in which the mollusc was embedded (fig. 1). These form prominences 

 on the mould, which are all embraced and covered over by the turns of the spire. The 

 aperture has a subquadrate figure, much higher than wide, and expanded at the outer 

 side, where it acquires an increased extension by the hollow spines. Fig. 2 shows the 

 size and form of the aperture, the squareness of the same, and the numerous ridges that 

 cross the area between the larger folds. 



Dumortier has figured a large fragment of this species, which exhibits its specific 

 characters extremely well, including the structure of the tubercles as here described 

 from my specimen. 



The lobe-line is not visible either on the Erench or English specimens. 



Affinities and Differences. — This species resembles Aegoceras armatum, Sow., in its 

 general figure, but difi"ers from that form in having larger whorls ; more numerous 

 and smaller tubercles ; and a more quadrate aperture. 



