OrVEKTEBKATE PALAEONTOLOGY. 251 



ceous epidermis; beaks subcentral, or placed about half-way between the 

 middle and the front ; hinge with two thin teeth in the right valve, and three 

 in the left, the middle one of the latter bifid; anterior adductor-scars elon- 

 gated very obliquely forward and downward, and placed near the anterior 

 upper margin; pallial sinus very small. 



The animal of the existing species of this genus has the siphons very 

 short, and the lobes of the mantle protected by a wrinkled epidermis. The 

 pedal opening is terminal, and the foot straight and truncated. 



Species apparently belonging to this genus are found in Cretaceous 

 rocks; but, owing to the fact that we kuownothing of their hinge-characters, 

 and rarely see any traces of their pallial line, we cannot be positively sure 

 thai they belong here. The genus was represented during the Tertiary 

 epoch; while some three or four species are known to exist at the present 

 time, and are found inhabiting muddy estuaries. 



This seems to be a rather small group, that was probably not much, 

 if any, more abundantly represented during former geological periods than 

 at present. It is most nearly related to Pharus, but differs in the details 

 of its hinge-teeth, and in wanting the strong, oblique, internal ridge, seen 

 under the hinge in that genus, as well as in the structure of the animal. 



Pharella! Dakotensis, m & h 



Plate 1, fig. 3. 



Solen? Dakotensis,Meek and Hayden (1857), Proceed. Acad. Nat. Sci. Philad.j IX, 242. 

 Pharella? Dakotensis, Meek and Hayden (Oct., 1860), ib., XII, 125.— Gabb (1861), Synop. Moll. Cret., 

 164.— Meek (1864), Smithsonian Check-List Cret. Fossils, 15. 



Shell elongated and narrow, rather compressed ; dorsal and ventral mar- 

 gins nearly straight and parallel; posterior extremity very narrowly rounded, 

 and apparently only moderately gaping ; beaks scarcely distinct from the 

 dorsal margin, and located nearly or quite centrally. Surface of cast retain- 

 ing faint traces of concentric marks of growth ; cardinal margin of cast show- 

 ing a very obscure sulcus along its entire length, both before and behind the 

 beaks. 



Length, about 1.55 inches ; height, 0.35 inch ; convexity, 0.20 inch. 



This shell resembles Pharella? equalis (= Solen equalis, d'Orbigny, 

 Paleont. Fr., Terr, (.'ret., Ill, pi. 350, figs. 5 et 6), more than any species I 

 have seen figured in works on palaeontology; but it is proportionally nar- 

 rower, or more depressed in form. 



