128 MONOMYArtlA. 



an external flattened cardinal space divided by a longitudinal 

 groove, and increasing with the age of the animal; hinge 

 vk'ith 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— VLlCkTUL A.— Lamarck. 



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.— PECTEN.— Zawm/cA. 



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 variiis. — 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. 



