435 



MONOGRAPH OF THE GENUS 

 PLICATULA. 



Class, Conchifera. Order, Monomyaria. Fam., Pectinides. 



Gen. Char. "Testa insequivalvis, inarticulata, basi {apex) at- 

 tenuata; margine supero {ventral') rotundato, subplicato; natibus 

 insequalibus ; areis externis nullis. Cardo dentibus duo validis in 

 utraque valva. Fovea intermedia bgainentum penitus internum 

 recipiens." Lamarck. 



The Plicatnlce are irregular, attached, bivalve shells, with one 

 muscular impression, and two teeth, locking into corresponding 

 hollows in each valve. In the attached valve, there is an external 

 triangular area at the hinge, but the ligament cartilage in 

 both valves is internal, being placed in a cicatrix between the 

 the teeth. All the species are, when freely developed, somewhat 

 triangular in form, and plicated ; but when hxed by a large portion 

 of their surface they lose their plicated character, and take the 

 form of substances to which they are attached, the plication only 

 commencing when the shell begins to grow freely. 



Between species so nearly resembling each other in their defor- 

 mities, as well as in their regular characters, it is impossible to 

 distinguished with certainty by any permanent characters. All we 

 undertake to do is to give a general account of the distinctions 

 sought to be established by others. 



1. Plicatula kamosa (pi. xc. f. 1 to 4.), Lam. Anim. s. 

 vert. vol. vi. p. 1 84. Plicatula gibbosa, ibid. Spondylus plicatus, 

 Linn. Gmel. p. 3298. 



P. testa, solida, crassa, ; plicis subrotundatis, geminatis ; colore 

 pallide fulvo, plicis lineis brevibus griseis vel rubris ornatis. 



The shell is solid ; the plicae are rounded, of average number, 

 and repeatedly doubled so as to branch out towards the margin. 

 The colour is arranged in short grey or red lines, on the top of 

 the folds. Lamarck gives America as the locality. Pig 4 is from 

 a specimen belonging to Mr. Cuming, from Lord Hood's Island. 



