HARPOCERAS EXARATUM. 441 



Affinities and Differences. — Harpoceras Levisoni has strong affinities with Harp, 

 difrons, still the build of the shells presents important points of difference. In Harp. 

 Levisoni the shelly laminae are thicker, and the style of costation different; the lateral groove 

 which divides the costse on the sides, and produces an outer costated half and an inner 

 smooth half in Harp, difrons, is absent in Harp. Levisoni. The form of the siphonal 

 area is likewise different, in Harp, difrons we have a small keel with grooves on 

 each side, whilst in Harp. Levisoni there is a thicker and more prominent keel, and 

 an absence of the two lateral furrows on each side of the carina. 



The morphology of the two species is likewise different. In early life the shell of 

 Harpoceras Levisoni likens it to that of an Arietites ; it has thick, straight costse, knobbed 

 at the angle, and a siphonal area with stout keel, and two lateral grooves, like Ariet. 

 obtusus ; all these characters, however, become obsolete as the evolution of the shell 

 proceeds. It is only by breaking up large specimens and finding the nucleus, that the 

 truth is revealed. When all these differences in the structural characters are fairly 

 considered, they are sufficient to justify the separation into two distinct species. 



Harpoceras exaratum, Young and Bird. PI. LXII, figs. 1, 2, 3. 



Ammonites exaratus, Young and Bird. Geological Survey of Yorkshire Coast, 



p. 266, 1828. 



— complanatus, d 1 Orbigny (non Bruguiere). Paleontol. Franchise, Terr. 



Jurass., p. 353, pi. 114, 1842. 



— exaratus, Simpson. Monogr. of the Ammonites, p. 32, 1843. 



— — — Fossils of the Yorksh. Lias, p. 72, 1855. 



— stjbplanatus, Dumortier. Depots Jurassiques, partie iv, p. 51, pis. x 



and xi, fig. 1, 2, 8, 1874. 

 Harpoceras exaratdm, Tate and Blake. Yorksh. Lias, p. 305, pi. ii, fig. 5, 1876. 



Diagnosis. — Shell discoidal, depressed, carinated, extremely involute, volutions three- 

 fourths concealed ; outer whorl in height about half the diameter of the disc. Costae 

 numerous, broad, distinct, simple, uniform, twice bent, and separated by neatly-cut 

 sigmoidal grooves ; sides of the whorls convex in the middle, inner margin sloping 

 towards the spiral line. Siphonal area narrow; keel sharp, prominent, and entire; 

 aperture elongated, lateral lappets short, slightly convex ; abdominal spine prolonged. 

 Umbilicus narrow, with its walls forming a re-entering angle. 



Dimensions. — Transverse diameter 95 millimetres ; height of the last whorl 45 

 millimetres ; width of the aperture 21 millimetres ; width of the umbilicus 18 millimetres. 



Description. — Harpoceras exaratum is a well-defined and very abundant form in the 

 Upper-Lias jet rock on the Yorkshire coast, where some large and beautiful shells are 



57 



