476 



MOLLUSCA 



PHYLUM VI 



Garditamera Conr. Elongated, siili-mytilifonn. Pleuromeris Conrad. Small, 

 equilateral, trigonal. Galyptogena Dall. Ovoid, smooth externally. Garditella 

 Smith. Small, with internal ligament. All Tertiary and Recent. Tlicadia Adams ; 

 Milneria Dall. Females with a shelly marsupium. Recent. 



Superfamily 5. CHAMACEA Geinitz. 



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

 groivth, and very unequal valves. Marine. 



Family 9. Diceratidae Dall.^ 



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

 projections which a^re prolonged into the umbonal cavity below the hinge plate ; valves 

 grotesquely distorted, sub-equal, with p)i'olongcd and tim'stcd umhones, or the free valve is 

 reduced to an opercular form, spira.l, and even concave; the teeth often reversed relatively 

 to their situation in Ghama. Jura and Cretaceous. 



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

 Mesozoic, and in .turn has given rise to a branch that has survived to the present day 



Dlccras arictinwni Lam. Coral-Raa; ; 

 St. Mihiel, Meu.se. s/.,. 



A, Diceras arietinum hum. Coral-Rag; St. Mihiel, Meuse. 

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

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

 adductor .sear.s ; c, Ma.ior cardinal ; d, Socket for left anterior 

 cardinal ; I, Ligamentary groove ; s, Buttres.s ridge before 

 po.sterior adductor scar. 



(Chamidae), as Avell 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 



1 For the Chamacea and Rudistae, NeumajT proposed the term Pachydonta. For special litera- 

 ture see : Zittel, K. A. von. Die Bivalven der GosaugebiMe. Deukschr. Akad. Wis.s. Wien, 

 1864, voh xxiv.—Oemmellaro, G. G., Caprinellidi della Ciaca dei dintorni di Palermo, 1865.— 

 Munier-Ghalmas, E. , Prodrome d'une classification des Paidistes. Journ. deConchyl., 1873, vol. xxi. 

 — White, a. A., Bull. U.S. Geol. Surv., No. 4, 1884 ; No. 22, X'i^b.—DouvilU, H., 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.— (Zi Stefano, G., Studii stratigrafici e paleontologici sul systema cretaceo di Sicilia. 

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

 Palaeout. Ital., 1898, vol. \v. — Fuiterer, K., Die Obereu Kreidebildungen der Umgebung des Lago 

 di Santa Croce. Palaeont. Abh., 1892, n.s. vol. M.—Bohm, G., Beitriige zur Kenntnis der Kreide in 

 den Siidalpen. Palaeontogr., 1894, vol. xU.—Douville, JT., Eltudes sur les Eudistes. Mem. Soc. 

 Geol. France. Palrontologie, i. iii., 1890-96.— Paro/(a, 0. F., Sopra alcune Rudiste Senonianc 

 dell' Appeniuo ineridiouale." Mem. Accad. Torino, 1900, ser. 2, vol. l.—Paqider, V., Les Rudistcs 

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

 tion et revolution des Hippuiites. torn, cii., 1903, and vol. xxi. p. 506, 1891. 



