358 CEETACEOUS PELECYPODA 



IX. Order,— MYTILACEA. 



The shells referable to this order represent the Hetero-myaria of authors. 

 All the genera are characterized by a marked inequality of the adductor muscles, 

 the anterior decreasing in size, until it becomes almost obsolete, while the posterior 

 one increases in nearly the same ratio, finally assuming a semilunar or an elongate 

 and curved shape, this being mostly characteristic for the OSTRUA CHA^ appa- 

 rently the lowest organised of all Pelecypoda. 



The animals have the mantle margins mostly open, posteriorly partially united 

 and provided with a special anal tube or slit ; only in very few, almost exceptional, 

 cases is there also a separate branchial opening present. All the species possess 

 one pair of palps and one of gills on either side ; the foot is generally small, rarely 

 elongated and thin, but mostly supplied with a strong byssal gland ; the pedal and 

 byssal muscles are always well developed. 



The shells have generally a large size and are externally covered with a horny, 

 scaly or papillose, epidermis, internally they are more or less pearly ; the ligament 

 is long, often linear, usually sub-external ; hinge line long, with a few irregular 

 tubercular teeth or without them ; pallial line entire, exceptionally sinuate ; mus- 

 cular scars always unequal. 



The different forms can be classed in four families, Peasinib^^ Mytilibm^ 

 FiNNiB^i and A viculidjs. Their respective relations to the more highly organised 

 Pelecypoda are indicated by the order in which they follow each other, and will 

 be alluded to in more detail subsequently. Some conchologists class the Mytilid^ 

 before the ABC AGE A, and place the Aviculid^ in the OSTBBACJEA. There 

 can be little doubt that this classification is in some respect justifiable, for the 

 Mytilidm possess an organisation of the animals which agrees in some points, — as, 

 for instance, in the presence of a special anal opening — with the L TIC IN A OJEA^ but, 

 on the other hand, their shells indicate a lower grade in the system on account of the 

 want of regular hinge-teeth, the marked inequality of the muscular scars, and some- 

 times of the valves themselves, &c. These characters are to a certain extent common 

 to all the MYTILA CJEA, which form, I believe, a similarly natural group, as do 

 the ABCACUA, However, as the apparently highest organised genera of the 

 former order, JPhaseolicama and Dreissena, can hardly be placed higher in the 

 system, than the NuculanidjE of the other order, while the lowest forms of 

 the MYTILACEA, the Aviculibm, are decidedly much more allied to the 

 OSTBEA CEA, than are the corresponding lowest forms of the AB CA CEA, I 

 think the classification here adopted is more natural than the one usually followed 

 in conchological treatises. 



Again, I do not think that a sufficient reason exists for the classification of 

 the Avioulidje: in the OSTREACEA, because these latter never possess an anterior 

 adductor, nor the peculiar horny epidermis of the shell, both of which are very 

 characteristic for all the MYTILACEA, 



Recent species of this order are not so numerous as the fossil which date 

 from the earliest fossiliferous deposits, and many of the smaller sub-divisions may 



