210 MOLLUSCA. LAMELLIBRANCHIA 



on the presence of one or two adductor muscles. Between 

 these two groups, however, there are numerous inter- 

 mediate forms in which the anterior muscle is smaller 

 than the posterior ; and, further, one genus (Dimya), 

 which in other respect agrees with some of the Mono- 

 myaria, possesses two adductor muscles. A third division, 

 the Heteromyaria, was established for the genera in which 

 the anterior adductor is small, but its limits are not well 

 defined, and moreover, in Mytilus it is found that whilst 

 in M. edulis both muscles are present, in the closely allied 

 species, M. latus, the anterior adductor is absent. Another 

 classification, which was suggested by Fleming, was based 

 on the presence or absence of siphons ; the two groups 

 being termed the Siphonida and the Asiphonida; the 

 former has been further divided according as to whether 

 the pallial line is entire (Integripalliata) or provided with 

 a sinus (Sinupalliata). Neumayr, Dall, and others have 

 divided the lamellibranchs into groups based largely on 

 the character of the hinge (p. 203) ; whilst the classifica- 

 tion brought forward by Pelseneer, is founded on the form 

 and structure of the gills. The divisions which are pro- 

 visionally used in the following pages are similar to those 

 proposed by Neumayr. 



1. Hinge taxodont. Two nearly equal adductor 

 muscles. Siphons usually wanting. 



Nucula 1 (fig. 93 A). Shell equivalve, trigonal or oval, closed, 

 posterior side very short ; umbones directed posteriorly. Surface 

 smooth or ornamented. Interior nacreous. Margins of valves 

 smooth or crenulated. Hinge angular, with a median internal 

 triangular ligament-pit, and numerous sharp teeth. Adductor im- 

 pressions nearly equal. Pallial line simple. Silurian to present 

 day. Ex. N. hammeri. Lias ; N. dixoni, Bracklesham Beds. 



1 All the genera of Mollusca described are marine unless otherwise 

 stated. 



