738 LAMELLIBRANCHIATA. 



surface, and a broad pallial sinus. The genus is known by Cre- 

 taceous, Tertiary, and Recent species. 



In the singular genus Thetis, now usually included here, the shell 

 is gibbous and very thin, and there is an exceedingly deep, angular, 

 pallial sinus, which extends nearly to the beaks. The known species 

 of this genus are found in the Cretaceous rocks. Finally, the genus 

 Petricola, now usually regarded as the type of a separate family, 

 comprises Bivalves which differ from the Veneridce in their habit of 

 burrowing into rocks or sand, or in being fixed into crevices by a 

 byssus. The shell in this genus is thin and tumid, with a short 

 anterior side and a deep pallial sinus. The range of the genus is 

 from the Cretaceous period to the present day. 



Family 4. Cyrenid/e. — In this family the mantle is open in 

 front ; a single siphon is present, or, more usually, two more or less 

 united ; and the foot is large and tongue-shaped. The shell is sub- 

 orbicular, closed, with a thick epidermis and a concentrically-striated 

 surface, the hinge with cardinal and lateral teeth, and the ligament 

 external. The pallial line is simple or slightly sinuated. The 

 members of this family inhabit fresh or brackish waters, and their 

 remains are commonly found in estuarine or lacustrine deposits of 

 Mesozoic and Tertiary age. 



In the genus Cyrena (fig. 619) the shell is thick, and rounded 

 or subtrigonal, the beaks being commonly eroded. The hinge in 

 each valve has three cardinal teeth, and a single lateral tooth in 



front and behind. The forms 

 which are included under the 

 name of Corbicula differ from 

 Cyrena proper principally in 

 the fact that the lateral teeth 

 are elongated and are trans- 

 Fig 619— Cyrena antiqua. Eocene. Versely Striated. The oldest 



representatives of these two 

 types appear in the Jurassic rocks, and there are numerous living 

 forms, the Cretaceous and Tertiary rocks having also yielded many 

 characteristic species. 



The genus Sphczrium (Cye/as) comprises fresh-water forms in 

 which the shell is of small size, thin, more or less orbicular, and 

 nearly equilateral. The right valve has one cardinal tooth, often 

 bifid, and the left valve has two. The genus Pisidium differs from 

 the preceding in little except that the shell is inequilateral, the 

 anterior side being the longest. Both these types are now in ex- 

 istence, and the earliest fossil forms commence in the Eocene 

 Tertiary. 



Family 5. Ungulinid.e. — In this family the mantle-lobes are free ; 

 and the foot is long and vermiform, and does not secrete a byssus. The 



