HARPOCERAS COSTULA. 461 



Harpoceras costula, Reinecke. PI. LXXXII, figs. 5 and 6. 



Nautilus costula, Reinecke. Naut. et Argonaut, p. 68, tab. hi, figs. 33, 



34, 1818. 

 Ammonites costatulus, Schlotheim. Petrefactenkunde, No. 33, p. 78, 1820. 



— — Zieten. Versteiner, Wiirtemb., pi. 7, 1830. 



— radians costula, Quenst. Cephalopoden, p. 113, t. vii, fig. 11, 1849. 



— costula, Oppel. Die Juraformation, p. 247, 1856. 



Diagnosis. — Shell small, discoidal, compressed, carinated ; whorls two thirds involute ; 

 sides covered with fifteen to twenty straight, prominent ribs, which occupy the inner two 

 thirds of the whorl with wide, deep valleys between ; area smooth with distinct, elevated, 

 prominent keel ; inner margin not truncated ; aperture oblong. 



Dimensions. — (Fig. 6) Transverse diameter 26 millimetres ; width of umbilicus 8 

 millimetres; height of the last whorl 10 millimetres; (fig. 5) transverse diameter 33 

 millimetres ; width of umbilicus 8 millimetres ; height of last whorl 1 1 millimetres. 



Description. — This small shell belongs to the same horizon as the preceding species ; 

 it is always diminutive in size, seldom attaining more than one inch in diameter and 

 exhibiting then a portion of the body-chamber. The shell is compressed ; the whorls 

 are more than two thirds involute, and the sides ornamented with from fifteen to twenty 

 straight, prominent ribs, which occupy only two thirds of the inner side of the whorl, and 

 leave the upper portion of the side and area smooth with a strong prominent keel standing 

 in the centre of the area. The umbilicus is narrow, and the inner whorls smooth and 

 without costae. The aperture is oblong and much invaded by the return of the spire. 



Affinities and Differences. — This Ammonite resembles some of the ribbed youthful 

 forms of Harp. Aalense ; still, the prominent ribs at wide intervals apart, with the keel 

 entire and prominent, are characters that are wanting in the usual forms in Harp. 

 Aalense. 



Locality and Stratigraphical Position. — The specimen I have figured (PI. LXXXII, 

 fig. 5) was collected from the fine grey marl in the Kocher Thai, by Wasserelfingen, 

 Wurtemberg, associated with Harp. Aalense and Harp, radians^ all forms that charac- 

 terise the Lyt. Jurense-zone in Germany. The same form with less coarse ribs (fig. 6) 

 occurs in the Lyt. Jurense-zone at Frocester Hill, where it has long been considered to 

 be the inner portion of Harp. Aalense, with which it is found associated. 



