494 F. R. Cowper Reed — Fossils from Haverfordwest. 



poorly preserved material from the British formations, and con- 

 sequently the new or revised generic terminology adopted by him 

 cannot in many cases be employed without some element of 

 uncertainty. 



In the present case there are three genera which have some 

 external points of resemblance to this Slade form. Orihodesma 

 resembles our shell in general shape, the gaping of both ends, and the 

 position of the beaks and muscles, but it is stated to have a broad mesial 

 sulcus which is certainly absent in the Slade species. On the other 

 hand, 0. canalicidatum, Ulrich,^ has only a very shallow sulcus, and 

 possesses obscure posterior radii, a kind of ornamentation apparently 

 wanting in other members. 0. minnesotense, Ulrich,^ also appears 

 to be allied to our form, judging from the figures and description; 

 the shape of the shell is nearly identical, and the mesial sulcus 

 appears to be absent. Endodesma ^ has a more or less defined lunula 

 in front of the beaks, but the mesial depression is strong and the 

 basal margin consequently sinuate ; however, the general external 

 aspect of some species is not very dissimilar from Orthodesma. 

 Whiteavesia ( — Actinomya) * has tightly closing valves and no 

 lunule, but a radial oi'namentation on the surface of the shell is 

 sometimes well marked. 



On the whole, of the above three genera our species seems most 

 probably to belong to Orthodesma. The external ornamentation 

 recalls Nuculites poststriahis, Emmons,^ of the Trenton and Hudson 

 River groups, but the shell is much shorter and higher, the beaks 

 are subanterior, and the umbonal ridge stronger and more prominent. 

 Modiolopsis postlineata, McCoy," with a similar ornamentation, is 

 a shorter shell, with almost terminal beaks and a more abruptly 

 truncated posterior end, but the type is imperfect, and it may be 

 allied to this new Slade species. 



Ctenodonta sladensis, sp. nov. (PI. XXIV, Fig. 12.) 



Description. — Shell subquadrate-ovate or subrhomboidal, about 

 one and a half times as long as high, thin, compressed ; anterior end 

 rounded ; posterior end obliquely truncate above, rounded below, 

 meeting hinge-line at about 45° ; inferior margin broadly rounded ; 

 cardinal margin behind beaks straight and about half the length of 

 sliell ; beaks small, not prominent, directed forwards, situated at 

 about one-third the length of the shell from anterior end, which 

 is slightly excavated below them. Umbonal ridge sharply marked, 

 slightly arched ; cardinal slope above it flattened, compressed. 

 Teeth behind beak obliquely inclined, transverse, 10 or more in 

 number. Surface of shell regularly marked with very fine con- 

 centric striae. 



' Ulricli: op. cit., p. 520, pi. xxxvii, figs. 7-11. 

 - Ulrich: op. cit., p. 517, pi. xxxvii, figs. 12-14. 

 ^ Ulrich: op. cit., p. 525. 

 •* Uli-icii: op. cit., pp. 513, 628. 



* Emmons: Geol. Kep., 1842, p. 399, fig. 3. Hall: Palpcont. New York, vol. i, 

 pp. 151, 301, pi. xxxiv, figs. 2fl!, b; pi. Ixxxii, figs. lOff, b. 

 " McCoy: S}Tiops. Palteoz. Foss. Woodw. Mus., p. 268, pi. ii, fig. 22. 



