128 MONOMYARIA. 



an external flattened cardinal space divided by a longitudinal 

 groove, and increasing 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. 



Spondylus longispina. — The Long-spined Spondylus. 

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

 thickly spined; of a reddish colour; alternate spines, arcu- 

 ated and tongue shaped; umbones orange. Four inches 

 long. Inhabits the Indian ocean. 



Genus 17— PLICATULA. —ZamarcA. 



Generic Character Shell inequivalve, without ears, 



attenuated at the base, rounded and plaited at the upper 

 margin; beaks unequal and entire; hinge with two strong 

 teeth in each valve, with a central cavity for the ligament, 

 which is internal. 



PUcatula ramosa. — The Branched Plicatula. Plate 

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

 gitudinal 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.— PECTE'N.— Lamarck. 



Generic Character. — Shell free, regular, inequivalve, 

 eared; the lower margin transverse, straight; beaks con- 

 tiguous; hinge without teeth ; ligament internal, placed in 

 a triangular cavity. 

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



Pecten varius. — The Speckled Pecten. Plate XV. 

 fig. 5. Ears very unequal; having about thirty-two 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. 



