HARPOCERAS ACUTUM. 



Harpoceras acutum, Tate. PI. LXXXII, figs. 7 and 8. 



Ammonites " falciferen," Quenstedt. Jura, p. 174, tab. xxii, fig. 31, 1858. 

 Ammonites serpentinus, Beesley. Geol. of Banbury, p. 10, 2nd ed., 1873. 



— acutus, Tate. Geological Magazine, decade ii, vol. ii, p. 204, 1875. 



Diagnosis. — Shell discoidal, carinated, and compressed; whorls one half involute, 

 sides convex, covered with numerous small, single, close-set, biflexed radii, which impart 

 a neatness and compactness to the whorls ; siphonal area convex, with a thick, promi- 

 nent keel, inner side sloping gently to the spiral suture ; aperture oval. 



Dimensions. — Transverse diameter 31 millimetres ; height of last whorl 13 milli- 

 metres; width of umbilicus 9 millimetres; height of aperture 12 millimetres; width of 

 aperture 8 millimetres. 



Description. — This neat little Ammonite was collected by my friend Professor 

 Quenstedt near Diirnau, associated with Am. globosus in a bed of Lias equivalent 

 to the zone of Am. spinatus. It was subsequently discovered in England by Mr. 

 Beesley, E.G.S., and catalogued 1 as Am. serpentinus in his ' Sketch of the Geology of 

 Banbury.' Its difference from the young forms of that species was, however, noticed by 

 Professor Ralph Tate, who entered this Ammonite in his " Notes on some New 

 Liassic Fossils," 2 under the name Ammonites acutus. Our author writes : — " This 

 species has some resemblance to, but is obviously distinct from, A. serpentinus. No 

 specific name has yet been applied to the species represented by Quenstedt's figure, 

 though one or two authors have sought to include it under certain new species described 

 by them. A. pseudo-radians, Reynes makes a near approach to it, but the whorls are 

 more embracing in the present species, which I call A. acutus on account of its sharp, 

 elevated keel. As Sowerby's A. acutus is now recognised as belonging to A. marga- 

 ritatus, the specific name is free to be re-applied." 



This is certainly a very elegant little Ammonite belonging to the radians group, 

 is remarkable for its small, thin, lustrous shell, and convex sides closely covered 

 with simple, hair-like, bi-flexed striae, which commence at the umbilicus, ascend over 

 the sides, and pass over the thick, prominent keel, certainly large in proportion to so 

 small a shell. The radii are very delicate in young shells, and become more prominent 

 in those of larger growth, such as the specimen I have figured and enlarged two 

 diameters. The umbilicus is narrow, and all the inner whorls are exposed. The aper- 

 ture is ovate, and cut out below by the return of the spire. 



Affinities and Differences. — Harpoceras acutum certainly belongs to the radians 



1 ' Sketch of the Geology of the Neighbourhood of Banbury,' 2nd ed., 1873. 



2 ' Geological Magazine,' decade ii, vol. ii, p, 204, 1875 



