162 CLASS I. TROPIOPODA. ORDER II. UNIMUSCULOSA. 



authors of the present day are unwilling to admit of any distinction. 

 It is certainly difficult to trace a distinct umbonal area in some Spondyli, 

 but still this peculiarity of growth is remarkable in most of the species ; 

 there is moreover a certain character about the Plicatulse by which they 

 may be readily distinguished. 



The shell of Plicatula may be described as being attached, irregular, 

 inequivalve, attenuated towards the base, but not auriculated ; the upper 

 margin is round and somewhat plicated ; the umbones are unequal, and 

 do not form any external area. The hinge consists of two teeth in each 

 valve strongly interlocking with each other ; and the ligament is internal, 

 inserted in an intermediate pit. The muscular impression inclines towards 

 the side, and is very strongly marked. 



Examples. 



PL CXV. Fig. 1 and 2. 



Plicatula gibbosa, Lamarck, Syst. Ann., p. 132. Chemnitz, Conch., 



vol. vii. pi. 47. f. 479 and 480. 

 Spondylus plicatus, Linnaeus. Deshayes. 

 Plicatula ramosa, Lamarck (Anim. sans vert.). 



PL CXV. Fig. 3. (fossil.) 



Plicatula spinosa, Sowerby, Mineral Conch., pi. 245. Genera of Shells, 



No. 3. Enc. Meth., pi. 175. f. 1 to 4. 

 P lacuna pectinoides, Lamarck. 

 Plicatula pectinoides, Deshayes. 

 Harpax (pectinoides?), Parkinson. Gray? 

 Harpax Parkinsonii ? De Blainville. 



SPONDYLUS, Linmeus. 



Testa affixa, inaequivalvis, subirregularis, plus minusve auriculata, extiis 

 vel rigida vel spinosa ; umbonibus inaequalibus, valvae inferioris area 



