152 GASTEROPODA OF THE INFERIOR OOLITE. 



shorter, and to be more coarsely ornamented in the zones of Am. Humphriesianus- 

 and Parlcinsoni. 



Description of var. spinicostata : 



Length of a fair-sized specimen . . 35 mm. 



Width . . . . .7-5 mm. 



Ratio of body- whorl to entire length . . 31 : 100. 



Spiral angle . . . .15°. 



Shell elongate, strongly turrited. Spiral angle nearly regular ; whorls about 

 fifteen in number, subconvex in the apical region, but becoming flatter and 

 somewhat pinched anteriorly so as to increase the turriting of the shells. The 

 ornaments consist of about eight fine spiral lines, which in the latter whorls 

 assume the character of spiral belts, with a considerable interspace ; subordinate 

 spiral lines may be detected, especially in the more mature whorls. The upper- 

 most one is strongly tuberculate at the points of intersection with the costae, 

 which are numerous. The costae are rather fine, and extend from suture to 

 suture, decussating with the spirals so as to present a somewhat granular appear- 

 ance at the points of intersection. 



The body-whorl is less than one third the entire length of the shell, the shape 

 and ornamentation are similar in character to the whorls of the spire, with, 

 however, a tendency for the costae to become more arcuate. The base is tolerably 

 full and marked with strong spiral striae, which are partially decussated by a faint 

 continuation of the costae. The aperture is ovate-elongate ; the outer lip thin 

 (always broken away). There exists a kind of rudimentary posterior canal. 

 Columella excavated and but slightly encrusted. Anterior canal considerably 

 produced, but slightly reflexed. 



The following sub- varieties may be noted. Fig. 8 b is less subulate, and 

 shorter. The specimen figured has suffered slightly from wear or solvents, so that 

 the spiral fines are less distinct. The costae are scarcely decussated in conse- 

 quence. This is a medium-sized specimen and greatly resembles some of the 

 larger forms of G. sub scalar if orme from the Bayeux-beds. Fig. 8 c represents a 

 very elongate form, with fine ornamentation. In Fig. 8 d the proportions are as 

 usual, but that which constitutes some difference is the twist in the costae, and 

 their marked inclination from right to left. In this specimen the spiral lines on 

 the costae are entirely obliterated. In Fig. 8 e the costae are stouter and less 

 numerous than usual, with great salience of the spinous row of nodules on the 

 shoulder of each whorl. The traces of spiral ornamentation are still more difficult 

 to find. This sub-variety leads up to Cerithium subglabrum. 



Relations and Distribution. — The most typical forms of G. spinicostatum are to 

 be met with in the Sowerbyi-bed of Bradford Abbas, where it is one of the most 

 abundant fossils, though not always in the best state of preservation. It is 



