476 



MOLLUSCA 



PHYLUM VI 



Garditamera Conr. Elongated, sub-mytiliform. Pleuromeris Conrad. Small, 

 eqiiilateral, trigonal. Cahjidogena Dali. Ovoid, smootli extenially. Garditella 

 Sinitli. Siuall, witli internal ligament. All Tertiary and Kecent. Thecalia Adams ; 

 Milneria Dali. Females with a slielly marsupium. Eecent. 



Superfamily 5. CHAMACEA Geinitz. 



Garditian forms specialised for a sessile habit, usually with exceptionally spiral 

 growth, and very unegual valves. Marine. 



Family 9. Diceratidae Dall.^ 



Resemhling Ghama, but with the adductors usually borne on myophoric laminae, or 

 projections which o.re prolonged into the umbonal cavity below the hinge plate ; valves 

 grotesquely distorted, sub-equal, with prolonged and twisted umbones, or the free valve is 

 reduced to an opercular form, spiral, and even concave ; the teeth often reversed relatively 

 to their Situation in Ghama. Jura and Cretaceous. 



This family lias possibly been derived from the Megalodontidae of the Paleozoic and early 

 Mesozoic, and in .tuvii has given rise to a branch that has siu'vived to the present day 



Fig. 765. 



Diceras arietinum Lam. Coral-Rag ; 

 St. Mihiel, Meuse. 2u 



Fio. 766. 



A, Diceras arietmum Lam. Coral-Rag; St. Mihiel, Meuse. 

 Fixed left valve, 2/3. B, D. zitteli, Mun. - Chalm. Tithonian ; 

 Stramberg. Right valve, 2/3. a, a', Anterior and posterior 

 adductor sears ; c, Major cardinal ; d, Socket for left anterior 

 Cardinal ; l, Ligamentary groove ; s, Buttress ridge before 

 posterior adductor scar. 



(Chamidae), as well as to others that became extinct at the close of the Mesozoic. In all cases 

 the forms in which the umbo of the free valve is coiled have preceded more specialised forms 



^ For the Ghamacea and Rudistae, Neumayr proposed the term Pachydonta. For special litera- 

 ture see: Zittel, K. A. von,J)\Q Bivalven der Gosaugebilde. Denkschr. Akad. Wiss. Wien, 

 1864, vol. xxiv.—Gemmellaro, O. G., Caprinellidi della Ciaca dei dintorni di Palermo, 1865.— 

 Munier- Ohalmas, K, Prodrome d'une Classification des Rudistes. Journ. deConchyl., 1873, vol. xxi. 

 — White, O. A., Bull. U.S. Geol. Surv., No. 4, 1884 ; No. 22, 1885. —Bouville, IL. Several papers in 

 Bull. Soc. Geol. France [3], xiv. p.. 389 ; xv. p. 756 ; xvi. p. 699 ; xvii. p. 627 ; xviii. p. 324 ; 

 1886-90. — di Stefano, G., Studii stratigrafici e paleontologici sul systema cretaceo di Sicilia. 

 T. Gli Strati con Caprotina. Palermo, 1888. II. Calcari con Polyconites di Termini-Imerese. 

 Palaeont. Ital., 1898, vol. iv. — Futterer, K., Die Oberen Kreidebildungen der Umgebung des Lago 

 di Santa Croce. Palaeont. Abb., 1892, n.s. vol. ii.—Böhvi, G., Beiträge zur Kenntnis der Kreide in 

 den Sitdalpen. Palaeontogr., 1894, vol. x\i.—Douville, H., Etudes sur les Rudistes. Mem. Soc. 

 Geol. France. Paleontologie, i. iii., 1890-96.— /^arowa, G. F., Sopra alcune Rudiste Senoniane 

 deir Appenino meridionale. Mem. Accad. Torino, 1900, ser. 2, vol. I.—Paquier, F., Les Rudistes 

 urgoniens. Mem. Soc. Geol. France, Paleont., 1903, vol. xi.—Toucas, B., fetudes sur la Classifica- 

 tion et l'evohition des Hippurites. tom. cit., 1903, and vol. xxi. p. 506, 1891. 



