296 THE LIAS AMMONITES. 



the marginal angle ; siphonal area squarish with a thick prominent keel, and two deep 

 lateral sulci. Outer lamina of the shell ornamented with elevated longitudinal and 

 transverse lines, growing at some distance apart and forming decussating striae with a 

 series of elevations at the points of intersection ; aperture oblong, subquadrate, deeper 

 than wide ; siphonal area narrow and bisulcate ; dorsal margin widest where it embraces 

 the penultimate whorl. 



Original type specimens of Sowerby — the largest shell (PI. XXII, figs. 1, 2) : — 



Dimensions. — Transverse diameter 125 millimetres; width of umbilicus 40 milli- 

 metres ; height of aperture 50 millimetres ; greatest dorsal width 43 millimetres. 



Smaller shell (PI. XXII, figs. 3 — 6), the same as figured in the ' Mineral Conchology ' : 

 transverse diameter 73 millimetres; width of umbilicus 25 millimetres; height of 

 aperture 30 millimetres; greatest siphonal width 25 millimetres. 



Description. — The shell is discoidal, compressed ; tricarinate on the siphonal area, 

 and its sides ornamented with thirty simple ribs, slightly bent in the inner half, 

 curved forward, and much less prominent as they approach the angle ; the shell is 

 bevelled away toward the abdominal side, and here the ribs entirely vanish. The 

 siphonal area is narrow from the bevelling of the shell ; the keel is thick and prominent 

 (figs. 2 and 5), with a deep sulcus on each side. The spire is composed of four compressed 

 whorls, which cover more than a third, about -^jth of the embracing whorl, so that the 

 entire umbilicus is exposed. The surface of the outer lamina of the shell is ornamented with 

 distant, elevated, longitudinal, and transverse lines, and the decussating striated surface 

 thus produced forms small tubercles at the points of intersection, which impart a remarkable 

 feature to this and the allied species, Arietites obtusus. These decussating striae are very 

 well shown in the siphonal areas of both the figured specimens ; the aperture is oblong, 

 subquadrate, compressed at the siphonal side, with a sinuous margin, and enlarged at 

 the umbilical side, where it embraces the preceding whorl. 



The lobe-line, very well shown in fig. 3, forms one siphonal, and three lateral lobes, 

 with four saddles on each side. The siphonal lobe is one third longer and wider than 

 the superior lateral lobe, which is small, and the lower lateral and the auxiliary are so 

 likewise. The lateral saddles are very large, and much exceed in size the siphonal and 

 auxiliary saddles, as is very well seen in figs. 3 and 6. In none of the specimens that 

 have passed through my hands have I seen the form and complexity which d'Orbigny 

 has figured in his plate xlv. 



The Anaptychus, which I found in the body-chamber of a large specimen at Lyme 

 Regis, has an uneven, compressed, bell-shaped figure ; it has a horny texture, and 

 much resembles the Anaptychus of A. semicostatus found very near the obtusus-zone. 



Affinities and Differences. — Arietites stellaris very much resembles^, obtusus (Plate XXI) 

 in the general form and punctated character of its shell' structure ; it differs, however, 

 from that Ammonite in having a more compressed shell, bevelled away toward the 

 margin, with more numerous ribs vanishing as they approach the angle, and leaving no 



