112 MONOMYAIRA. 



summit of the lower valve not being recurved, though much ad- 

 vanced beyond the upper valve. 



Genus 16.— S POND YL US. -Zinnaus. 



Generic Character. — Inequi valve, adherent, eared, spined, and 

 rough, with unequal beaks ; lower valve with an external longi- 

 tudinal groove, which increases with the age of the animal ; 

 hinge with two very strong hooked teeth in each valve, and a 

 central cavity for the ligament, communicating at its base with 

 the external groove ; ligament internal. 



Spondijlus longispina. — The Long- Spined Spondylus. 

 Plate XV. fig. 2. Longitudinally sulcated and ribbed, thickly 

 spined ; of a reddish colour ; alternate spines, arcuated and 

 tongue shaped ; umbones orange. Four inches long. Inhabits 

 the Indian ocean. 



Genus 17.— PLICA TULA — Lamarck. 



Generic Character.— Inequivalve, without ears, attenuated at 

 the base, rounded and plaited at the upper margin ; beaks une- 

 qual and entire ; hinge with two strong teeth in each valve, with 

 a central cavity for the ligament, which is internal. 



Plicatula ramosa. — The Branched Plicatula. Plate 

 XV. fig. 4. Oblong, trigonal, very thick; with strong longitu- 

 dinal plaits ; brown, or yellowish-brown, with rust-coloured, 

 arrow-shaped markings ; inside white. One inch and a half 

 long. Inhabits the American seas. 



Genus 18— PECTEN.— Zaniarc/;. 



Generic Character — Free, regular, inequivalve, eared ; the 

 lower margin transverse, straight ; beaks contiguous ; hinge 

 without teeth ; ligament internal, placed in a triangular cavity. 

 It has two sections — * ears equal or nearly so — ** ears unequal. 



Pecten varia.— The Speckled Pecten. Plate XV. fig. 5. 

 Ears very unequal ; having about twelve ribs, covered with 

 transverse prickly suberect scales, variously clouded and 

 speckled with a variety of colours. Two inches and a quarter 

 long. Inhabits the European seas. 



