HARPOCERAS ANTIQUUM. 431 



Harpoceras antiquum, Wriglit^ nov. sp. PI. LVII, figs. 1 — 4. 



Biar/nosis. — Shell discoidal, compressed, and carinated, volutions one-half involute, 

 sides convex and flattened ; ribs formed of many biflexed strise, sometimes prominent, 

 and always well bent forward towards the aperture ; keel thick, prominent, and in parts 

 crenated. 



Dimensions. — Transverse diameter 180 millimetres; height of aperture 70 milli- 

 metres ; thickness 40 millimetres ; width of umbilicus 60 millimetres. 



Description. — This shell very much resembles Am. Normanianus, d'Orb., in its style 

 of ribbing ; but, as the specimens that have come under my observation are all fragmentary 

 and belong to a much larger and older specimen than d'Orbigny's figure, which was 

 drawn natural size, the difierence between my fragments and d'Orbigny's shell may be 

 only those of increased age rather than of specific distinction. Am. Normanianus is 

 much less involute, and has a wider umbilicus than Harpoceras antiquum. The discovery 

 of younger and better specimens of this shell will enable future observers to solve the 

 problem ; as I have no trace of a suture-line on any of the fragments, one of our best 

 guides is absent. 



The shell is much compressed, carinated, and provided with a strong keel ; the whorls 

 are nearly one-half involute, and are thickest at the spiral suture ; thence they diminish 

 towards the siphonal area, which is narrow and lanceolate and terminates in a thick keel. 

 The sides are traversed transversely by a number of unequal, highly flexed falciform ribs, 

 made up apparently of a number of striae, which unite in fasciculi, and form unequal, 

 obtuse costse on the sides. The keel is thick, cord-like, and prominent, and makes an 

 important element in the structure of the shell. The test is very well preserved in some 

 of the fragments, the transverse fracture of these exposes the crystalline structure of the 

 thin shell, which does not exceed one millimetre in thickness. The structure and falci- 

 form flexures on the sides of the whorls are well seen in two fragments, delineated in 

 PI. LVII, figs. 3 and 4. Both of these specimens have the shell preserved, so that we have 

 here the true character of the ribs, with their shelly covering transformed into ferrous 

 disulphide (PeS2), by which the microscope markings on the test are preserved. As the 

 shell increases in size (PL LVII, fig. 1) the ribs lose their prominence, the siphonal area 

 becomes thicker and rounder, and the keel is more defined. 



Affinities and Differences. — This Ammonite is nearly allied to Am. Normanianus, 

 d'Orb. It is, however, more regularly ribbed, the ribs remain single and do not bifurcate ; 

 the volutions are more involute, and the umbilicus wider than in that species. There is, 

 however, a very near affinity between the forms, which can only be satisfactorily settled 

 by a careful comparison of more specimens. 



Locality and Stratigrapldcal Position. — This Ammonite was collected by the Rev. J. 



