Maryland Geological Sukvey 55 



while all of the Adesmacea, Martesia, Pholas and Teredo, are initiated 

 before the beginning of the Cretaceous and long survive its close. 



The late Mesozoic affinities of the Gastropods are quite as obvious as 

 those of the Pelecypods. Avellana, the Opisthobranch genus which is 

 represented by the largest number of species, is restricted to the Cre- 

 taceous together with the closely allied Cinulia. Acteon and Ringicula 

 were well established in the Mesozoic, although they are more closely 

 identified with the Tertiary faunas. Haminea, Acteocina and Cylichna 

 are all modern types which had a meager representation in the late 

 Mesozoic. The Pleurotomids did not reach their culmination until the 

 Tertiary, although they were no insignificant factor in the Upper Cre- 

 taceous faunas. The Yolutidw are a very highly specialized group and the 

 genera referable to it are, for the most part, very restricted in strati- 

 graphic distirbution. Rostellites, Tolutomorpha and Liopeplum all were 

 initiated in considerable numbers during the Upper Cretaceous, but none 

 survived its close. The group of the Fulguridce, Fasciolariidm, and Fusidce 

 is represented by a number of highly specialized genera, prolific during 

 the Upper Cretaceous but apparently restricted to it, notably Pyropsis, 

 most closely allied to Tudicla, and Serrifusus of the Fulguridw, Pieslo- 

 ch ilus and Odontofusus of the Fasciolariidce, and Pyrifusus of the Fusidas. 

 Pugnellus, the single representative of the Stromhidw, is also restricted 

 to the Cretaceous, while Anchura of the Aporrhaidw occurs in the Jura 

 as well. The Cerithiidm and Scalariida occur hut very rarely. Laxi- 

 spira, the one genus of the Yermitidae which can be determined with 

 assurance, has not been recognized excepting from the Cretaceous. Other 

 members of the family probably occur, but it is difficult to separate them 

 from the tubiculous worms. The Turritellidw as represented by the type 

 species are remarkably prolific in the Mesozoic as well as in the Cenozoic. 

 Pseudomelania, the characteristic Mesozoic genus of the Pyramidellidw, 

 was rather more abundant during the early and mid-Mesozoic than near 

 its close. The occurrence of the Xenophoridae and the Solariidce is insig- 

 nificant. Gyrodes, the most abundant genus of the Naticidce in the fauna 

 under discussion, is restricted to the Upper Cretaceous, while Lunatia and 

 Amauropsis have a much wider range. The Trochidm are represented 



