193 



straight beaks ; the hinge lateral, without teeth ; the liga- 

 ment marginal, linear, very long, and rather internal. Re- 

 cent and fossil. — PI. vi. fig. 23. 



MalleacecB. — A marginal, sublinear ligament, either 

 simple or interrupted by rows of teeth, or crenulae ; the 

 shell lamellous. 



1. Crenatula. — A flat, lamellated, subequivalved, rather 

 irregular bivalve ; the hinge sublateral, linear, marginal, and 

 crenulated ; the crenulse in rows, callus hollowed into pits, 

 which receive the ligament; no groove for byssus. Kecent 

 and fossil. — PI. viii. fig. 22. 



2. Perna. — A subequivalved, flattish bivalve, with la- 

 mellar texture ; the hinge linear, marginal, formed of trans- 

 verse, parallel, sulciform teeth, not penetrating, and between 

 which the ligament is inserted ; a posterior groove, rather 

 gaping, for the byssus, under the extremity of the hinge, 

 with callous sides. Recent and fossil. — PI. viii. fig. 21. 



3. Malleus. — A rough, subequivalved bivalve, generally 

 elongated ; sublobated at the base ; with small divaricating 

 beaks ; the hinge edentulous ; an oblong, conical small pit 

 under the beaks, traverses obliquely the ligamental groove ; 

 the ligament, which is short and almost external, is inserted 

 in the sloping depression in each valve. Recent. — PI. vii. 

 fig. 3. 



4. Avicula. — An inequivalved, fragile, rather smooth 

 bivalve ; the base transverse and straight, with produced 

 extremities and caudiform anteriorly ; the left valve notched ; 

 the hinge linear, with a tooth in each valve beneath the 

 beaks ; the ligamental area marginal, narrow and grooved, 

 not traversed by the byssus. Recent and fossil — PI vii. 

 fig. 6. 



3. Meleagrina. 



This genus differs from avicula in being nearly equal 

 valved, of a roundish square figure, with no hinge tooth, 



c c. 



