60 SYEIAN MOLLUSCAN FOSSILS. 



has the lateral surface of its valves covered with few and wide lamellae ; 

 while those of our examples are numerous, narrow, and exceedingly thin, the 

 anterior edges only of the valves being rugose. The test is exceptionally 

 thick and massive, and in the single valve the muscular impression is a deep 

 ]}\t with an abrupt bounding wall on the front side. 



Neither Conrad, Lartet, nor Fraas records any species of fossil Pcrna 

 as occurring in Syria, nor did Coquand meet with the genus in Algeria, 

 where the prevailing formation is Cretaceous. It is worthy of note, too, 

 that Pictet and Carapiche failed to detect Perna3 in the Cretaceous of Sainte 

 Croix, Switzerland, that Meek found none in the Cretaceous and Tertiary 

 of the Upper Missouri country, tliat Ferdinand Roemer found none in the 

 Cretaceous of Texas, and that but a single species has been recognized 

 in the Cretaceous of all India. On the other hand, d'Orbigny gives five 

 species as belonging to the Cretaceous of France, and Reuss describes three 

 from the Cretaceous of Bohemia. Species are numerous in the Jurassic. 

 Coll. Bird. 



Locality and Position. — If Jurassic, as they have the appearance of being, 

 the fossils are probably from Mejdel esh Shems. 



Perna tetragona, sp. nov. 



Plato VI, fls. a. 



With the specimens described under the last title was a right valve, from 

 the thin lower and posterior margins of which small portions have been 

 broken away. This valve so closely resembles the others in its sliarp and 

 hooked beaks, in the narrow and delicate lamellte upon the lateral surltices, 

 and in the width, depth, and rugosity of the byssal sinus, thnt, taking into 

 account the wide range of variation which is observed in recent species of 

 Aviciiliclw, it is possible tliat this is only a variety of orlentatis. 



Yet the thin, flat, quadrangular form of the valve, the absence of any 

 deep or distinctly limited muscular impression, the hinge margin nearly 

 horizontal in direction, wliile that of the other specimens is abruptly sloping, 

 and the ligamental grooves narrower than their interspaces, constitute dis- 

 tinctive characters so striking, that, if found to be constant in specimens here- 

 after to be discovered, they will require the establisliing of a new species, to 

 which we have given provisionally the name placed above. Coll. Bird. 



Locality and Position. — Doubtless the same as for the last species. 



