662 ZOOLOGY SECT. 



monomyarian types (Pseudolamellibranchia) appear first in the 

 Carboniferous, and the Siphoniata not until the Triassic period. 

 The modern genus Area dates from the Upper Cambrian, and thus 

 furnishes as striking an example of a " persistent type " as some 

 of the Brachiopods. 



There seems to be little doubt that the Protobranchia, and 

 especially Nucula, exhibit the most primitive type of pelecypod 

 organisation, as indicated by the plume-like gills with separate 

 filaments, the simple nephridia, and the distinct cerebral and 

 pleural ganglia ; absence of concrescence is always a mark of low 

 or generalised organisation. The Filibranchia with imperfectly 

 united gill-filaments come next, and are divisible into two groups, 

 isomyarian with equal-sized adductors and heteromyarian with 

 more or less atrophied anterior and proportionally enlarged posterior 

 adductor; the latter group is to be looked upon as the more 

 specialised, and leads to the Pseudolamellibranchia (monomyarian) 

 in which the anterior adductor disappears completely in the adult, 

 while the posterior is immensely enlarged and assumes a central 

 position. Similarly the isomyarian Filibranchia lead to the 

 Eulamellibranchia, which are equal-muscled, but have the gill- 

 filaments united into a complete basket-work. In the Eulamelli- 

 branchia, lastly, there is a gradual series of stages from compara- 

 tively generalised forms with free mantle-lobes up to the highly 

 specialised species with large siphons. That the Pseudolamelli- 

 branchia and the siphoniate Eulamellibranchia are to be looked 

 upon as the highest members of the class, is indicated, not only by 

 morphological evidence, but by their comparatively late appearance 

 in time. 



SINUPALLIATE 

 EULAMELLIBRANCHIA 



INTEGRIPALLIATE 

 EULAMELLIBRANCHIA 



PSEUDO-LAMELLIBRANCHIA 



HETEROMYARIAN 

 FILIBRANCHIA 



ISOMYARIAN 

 FILIBRANCHIA 



PROTOBRANCHIA 



l-'i'.. ."lift. Diagram illustrating the mutual relationships of the Pelecypoda. 



