ARIETITES CROSSII. 283 



Arietites Crossii, Wright, nov. sp. PI. X, figs. 1, 2. 



Diagnosis. — Shell discoidal, large, compressed, sides with twenty-four ribs, narrow, 

 sharp, and trenchant, much more developed near the line of involution than in any 

 other part of their traject ; they first bend backward, then arch forward, and finally 

 vanish near the outer margin. They are thickest at the line of involution and become 

 gradually thinner and less developed towards the margin, which is bevelled away. 

 Siphonal area narrow, with a sharp, prominent keel, and a shallow sulcus on each side 

 thereof. 



Dimensions. — Transverse diameter of figured specimen 350 millimetres; height of 

 the last whorl 10 millimetres; diameter of the umbilicus 170 millimetres; amount of 

 involution one-third of the width of the whorl. 



Description. — This very remarkable Ammonite was discovered by the Rev. J. E. 

 Cross, F.G.S., in the Scunthorpe Ironstone of North-west Lincolnshire, in beds belonging 

 to the Upper Buc/ctandi-series, containing Arietites semicostatus, Aegoceras Boucaul- 

 tianum, and others. This new species attains gigantic size, as shown by several specimens 

 in Mr. Cross's Collection. The whorls have their greatest thickness near the line of 

 involution, from which they slope gently outwards toward the margin, so that the 

 siphonal area forms a narrow and contracted space, in which the sharp, prominent keel, 

 and two shallow sulci are found (fig. 2). The ribs likewise differ from those of other 

 congeneric species, being largest and thickest near the line of involution, where in other 

 forms they are most slender. They bend gently backward, then forward, diminishing 

 in size as they ascend and disappearing near the outer bevelled margin of the siphonal 

 area. The last whorl possesses twenty-five ribs, which have a uniform character through- 

 out. The lobe-line was not seen in any of the specimens that passed through my hands. 

 The narrowness of the siphonal area is very well exhibited in the capital drawing 

 of Plate X, fig. 2. 



The aperture of the shell is compressed — narrowest towards its outer margin and 

 widest at the line of involution of the spine, and the last whorl embraces nearly one half 

 of the penultimate one ; the ribs are likewise nearly equidistant, narrow, and trenchant, 

 and the curve they describe is uniform throughout all the whorls. 



Affinities and Differences. — This large Ammonite resembles A. BucMandi, but differs 

 from that well-known species in the shape of the whorls : those of A. BucMandi are 

 square, and as wide at the siphonal as at the umbilical margin, whereas in A. Crossii 

 they are contracted at the siphonal and large at the umbilical side ; the ribs, moreover, 

 in A. Bucklandi are small at the umbilical and large at the siphonal side, a character 



