GENERA OF THE SUBORDER PENTAMEROIDEA 



181 



Conchid'tum. They are the costatc equivalent of Pen- 

 tamcrus and agree in many other external details with 

 that genus. For example, they are distinctly trilobate, 

 the trilohation being produced by an elongate tongue 

 extending forward from both valves. This trilobation 

 is carried to its extreme in this genus in the species 

 R. trilohata (Kindle and Breger). 



Rhip'uiiurn is a contemporary of Pentamerus, and 

 probably lived before Conch'uiium arrived in American 

 seas. 



Internally the genus is like Pentamerus in the ar- 

 rangement of its septa. Externally the ribs are as a 

 rule implanted or split at the front. Other species 

 besides the genotype assigned to this genus are: 



ConchiJium multicostatum (Hall) 1860 



C. riysius (Hall and Whitfield) 1872 (see Hall and Clarke, 



Pal. N. Y., vol. 8, pt. 2, p. 235) 

 C. tenukoita (Hall and Whitfield) 1 872 

 C. triloiala Kindle and Breger 1904 



Genus CONCHIDIUM Linnaeus (1753) 1760 



PI. 28, figs. 1-6, 8-12, 17, 18, 20, 23, 26; pi. 29, 

 fig. 4; t. figs. 25, 28, 33 



Linnxus, Syst. Nat., ed. 12, vol. 2, 1760, p. 163. 



Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, 1893, p. 231, 



pi. 66, figs. 11-14. 

 Syn. Gyfidia Dalman 1828, K. Vet. Akad. Handl. for 



1827, pp. 93, 100, pi. 4, figs. la-g. 

 Syn. Antirhynchonella Quenstedt 1868-1870, Petref. 



Deutsch., vol. 2, pp. 231, 727. 



Genotype. — C. b'doculare Linnasus 1753, Mus. 

 Tess., pi. 5, fig. 8. 



Description. Exterior. — Subtriangular to sub- 

 trapezoidal in outline, hinge-line very narrow; lateral 

 profile biconvex, the ventral valve usually having the 

 greater convexity. Anterior commissure rectimargin- 

 ate, sulcate or uniplicate, fold and sulcus usually nar- 

 rowly pointed and uncurved; delthyrium covered by 

 a specialized "deltidium" ; dorsal beak obtusely pointed, 

 umbo more or less swollen. Shell thick in the posterior 

 region, costate or multicostate ; shell substance fibrous, 

 impunctate. 



Ventral interior. — Spondylium duplex elongate, sup- 

 ported by a duplex septum usually for more than one- 

 half its length. Septum exceedingly long. 



Dorsal interior. — Septal plates divergent, consisting 

 of three parts: (1) The inner plates which form two 

 chambers on each side of the interior, uniting laterally 

 with the walls of the valve. The sockets are excavated 

 in the antero-lateral extremities of these plates adjacent 

 to their junction with the wall of the valve. (2) 

 Outer plates. The structures noted above are sup- 

 ported by two diverging lamella; which unite with the 

 valve directly. The line of union is marked by a 

 thickening. (3) Brachial supports. Two small plates. 



expanded at the free ends and united to the thickening 

 at the juncture of the outer plates and the inner plates. 

 The brachial processes extend into the cavity of the 

 valve, in some specimens, for a distance greater than 

 the length of the septal plates. At their distal extremi- 

 ties they curve laterally. Between the septal plates is a 

 low median septum, dividing the adductor impressions. 

 Geologic range. — Late Silurian. 



American Species 



Conchidium arcticum Holtedahl (Rept. 2d Norw. Arctic 

 Exped., "Fram," 1898-1902, No. 32, 1914, p. 5) 

 C.colletti (Miller) 1891 

 C. crassoradius (McChesney) 1861 

 C.cumberlandicum Prouty 1923 

 C. exfonens Hall and Clarke 1895 

 C. greenii Hall and Clarke 1893 

 C. lajueatum (Coniid) 185 5 

 C. lindenense Foerste 1903 

 C.littoni (Hall) 18 59 

 C. nettelrothi Hall and Clarke 1 893 

 C. obsoletum Hall and Clarke 1895 

 C.occidentde {WiSS) 1852 

 C. scofarium Hall and Clarke 1895 

 C . unguiforme (Ulrkh) 1886 



? C. georgits Hall and Clarke 1895 (may be Gyfidula) 



European Species 



Conchidium biloculare Linnaeus 1 760 

 C. miinsteri Kiaer 1901 ? 

 C. tenuistriatum Walmstedt 



Distinguishing characters. — The features of 

 Conchidium that distinguish it from the other Pentam- 

 erinas are the strongly multicostate exterior, usually 

 strongly biconvex valves, and extended beak, below 

 which there is a narrow interarea, a wide delthyrium, 

 and in some species a modified deltidium. Internally 

 Conchidium is like Pentamerus. 



Discussion. — It will be profitable to discuss several 

 morphologic features of Conchidium, i. e., the del- 

 tidium, the spondylium duplex, and the cardinalia. 

 Deltidial plates have been reported in many of the 

 pentamerids, such as Gyfidula. These plates have also 

 been termed pseudodeltidial plates by Booker.^' In 

 Conchidium, many specimens show these thickenings 

 along the margins of the delthyrium, but in addition 

 they have a concave or, in some instances, an elevated 

 plate covering the delthyrium. The two types — plates 

 and "deltidium" — may be observed on the same speci- 

 men. Obviously, then, the thickenings along the del- 

 thyrial margin are not true deltidial plates. This 

 statement will become more certain after a detailed 



=' lourn. Proc. Roy. Soc. N. S. Wales, vol. 60, 1927, 

 p. 134. 



