MANUAL OF THE MOLLUSCA. 



Ophileta, Yanuxem (p. 267), was founded on species of 

 Maclurea, witii yery slender whorls. Mr. Billings regards them 

 as distinct, and distinguishes them as follows : — " In Maclurea 

 the aperture is entire, and the whorls usually large, but in 

 Ophileta it has a sinus below and a notch above, while the 

 whorls are usually more slender. 



"In Maclurea crenulata (Billings) there is a sort of spiral band, 

 and also there are indications of a sinus in the lip on the flat 

 side, but they are only incipiently developed." — (Billings.) 



Steophosttlus, Hall. 



Etymology, sfrepho, I turn, and stylus, columella. 



Shell subglobose or ovoid ; spire small, body whorl large and 

 ventricose ; outer lip thin ; columella twisted or spirally grooved 

 within, not reflected ; umbilicus wanting ; aperture ovate or 

 transversely oval ; apparently related to Platy stoma. 



Distribution^ 10 species, Silurian. United States. 



Heliocbyptus, D'Orbigny, 1850. 



Shell depressed, orbicular ; whorls embracing ; umbilicated 

 on both sides ; aperture vertical, oval transverse. 



Distribution. H. pusillus, Coral Bag, Erance, Germany ; 

 11. radiatus, U. Gr. S. Blackdown, Mans. 



[Family Ianthinid-s:] 

 Includes — 

 Ianthina and Eecluzia (see p. 285). 



Family XII. — Calypth^ld^.* 

 Platyceras, Conrad, 1840 (see p. 277). 



Type, Pileopsis vetusta, Sowerby. 



Synonyms, Acroculia, Phillips, 1841; Orthonychia, Hall, 1843. 



Shell depressed, subglobose to oblique, subcorneal; spire 

 small ; whorls few, free or contiguous ; aperture more or less 

 expanded, often campanulated, entire or sinuous. 



Many species show a sinuosity of the strise, indicating a notch 

 in the margin of the aperture during the first stages of growth. 

 Mr. Hall has been unable to recognise the peculiar muscular 

 impressions which are characteristic of Pileopsis. Specimens of 

 some species show the expansion of the columellar lip, and its 

 partial or entire union with the volution, presenting all the 



* See p. 275. 



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