94 CRETACEOUS PELECYPODA 



a. Suh-faiinIu,—SOLENIN^. 

 Shell very lon^, with the beaks at, or near, the anterior end, gaping at 

 both ends ; hinge with few teeth ; ligament long, external on a thickened fulcrum, 

 pallial line truncate, at first prolonged from the posterior muscular impression 

 posteriorly in the direction of the longer diameter of the shell, and then moder- 

 ately but broadly insinuated at the end. Siphons of the animals short, imited 

 or separated only at their terminations. 



1. Solen, Linn., 1757. Shell straight, or nearly so, sub-cylindrical ; beaks 

 anterior, terminal or sub-terminal ; hinge with one tooth in each valve ; anterior 

 muscular impression long, beginning below the beaks and extending parallel to 

 the length of the shell ; pallial impression anteriorly with an oblique projection. 

 Several old authors, like Argenville, Klein, and others, used the name 

 Solen correctly, but not in the pure generic sense, having generally added some 

 other appellative to it; thus Klein writes Solen bivalois. Of all the numerous 

 species placed by Linn6 in the genus Solen, the name was reserved only for 

 S. vagina, which is the type. 



True species of Solen seem to occur ah-eady in palaeozoic rocks, but none of the 

 species, as, for instance, Sulen costaLus, Sandberger, and others have yet been found 

 perfectly well preserved. 



la. Solena, Browne, 175G. (Hijporjclla, Gray, "^ Flectosolen, Conrad, 18G6). 

 This name is used sub-generically for the designation of a section of Solen, (ohliqmis, 

 vaginatiis, &c.,) which have the beaks sub-anterior, and the anterior muscular 

 impression rounded. 



The distinction can hardly be considered as of much imj)ortance, inasmuch 

 as there are true Salens, which also have the anterior end somewhat produced. 

 Deshayes' figures represent several such species from the eocene of the Paris basin 

 (Paris Fossils, II cd., vol. I, pi. VII,) which, as far as form is concerned, belong to 

 Solena, but they all seem to possess the anterior muscular scar, elongated, hori- 

 zontal, as in typical recent species of Solen. 



lb. Flectosolen. Conrad (Am. Jom-n. Conch., II, 18G6, p. 103,) proposes this 

 name to include a certain number of eocene species (like S. gracilis. Sow., Sol. 

 angiistus, Des.), which he says are either straight or slightly curved ; the beaks are 

 nearly terminal, and an anterior furrow extends direct or oblique from them to the 

 lower margin, with a corresponding ridge within. This anterior ridge or rather in- 

 ternal depression corresponds to the sharpened edge of the foot when this is contracted 

 and then assumes a clavate shape ; it is the same as in typical Solen. The genus* seems 

 to me to have been proposed merely with the object of suiting Mr. Conrad's idea, 

 that every period which geologists call a formation was peopled by a distinct fauna. 

 2. Ensis, Schuhm., 1817. Shell slightly curved, beaks sub-terminal, hinge 

 with one tooth in the right and two teethf in the left valve, and a short ridge 



* Mr. Conrad appears to have since abandoned the name Plcctosolen, having found it to be based on untenable 

 characters (see Pal., Calif., II, p. 176). 



t H. and A. Adams, Gen.. II, p. 312, state that the hinsje is composed of two teeth in one, and three in the other 

 valve : this must be a typographical error, as their figure evidently shows the contrary-. 



