364 CRETACEOUS PELECYPODA 



Besides the fluviatile Dreissena there is another recent marine genus, Septife7\ 

 referahle to this sub-family. It agrees in the development of the hinge plate with 

 the former, but I have not as yet been able to observe the animal. The extinct 

 allied genera are Hojilomytllus, Anthracoptera, and Mijulina. 



h. CRENELLiN^. The animals of this sub-family have the mantle margins 

 ventrally mostly disunited, posteriorly in the branchial region produced and 

 ciliated, forming a kind of fold, which is superseded by a more or less produced 

 anal tube ; the palpi are of moderate size and narrowly elongated, the foot veiy 

 long, and exsertile, with a small byssal gland at the base. In Crenella the foot is 

 club-shaped at the end and the excurrent tube sessile ; in Modlolaria the byssal 

 gland is larger, often excreting a strong byssus, and the excurrent tube is elongated. 

 The pedal muscles are rather thin and long ; the anterior adductor thin, marginal, 

 anterior ; the posterior oblong and much stronger. 



The genera referable to this sub-family are Crenella, Dacridium, Modiolaria, 

 Arcoperna, and Ilijrina. 



c. MYTiLiNM. These also have the mantle margins nearly quite open, posteriorly 

 towards the upper end with a branchial fold, ciliated at the margin, and superseded 

 by a separate exhalant opening, not produced into a tube, but merely represented 

 by a slit ; the palpi are long, narrow, with pointed and often curled ends, on the 

 inner side striated ; the gills originate between the two palpi and extend posteriorly, 

 becoming gradually nan-ower ; their ends are somewhat turned upwards, but free, 

 not grown to each other or to the mantle, as is usually stated in conchological 

 treatises. At least in a number of species which I have examined, as, for instance, 

 3Iytiliis smaragdimis or Modiola tulipa, the gills are free, and if they are attached 

 in others, I would be inclined to look upon this more as an exception than as a rule, 

 because in many Aviculid^, which I examined on this point, I also found the gills 

 free posteriorly. 



There are only three genera referable to this sub-family, and they somewhat 

 differ in habitat and also in organisation. LUhodomus burrows in solid substances, 

 it has a small foot, and the byssus is in full grown specimens not developed ; the 

 anterior adductor is well marked, the pedal muscles moderate ; the posterior bran- 

 chial edges of the mantle become in some specimens greatly extended. Modiola 

 spins nests of foreign substances with its byssus, or lives in excavations of rock. 

 It has the foot very elongated, thin, sometimes slightly thickened at the end ; the 

 anterior adductor is narrow, thin, marginal ; the pedal muscles, specially the ante- 

 rior pair, arc rather thin and long. Mytilus has the labial palps almost half as 

 long as the whole animal ; the gills strongly bent upwards posteriorly ; the foot 

 short, sub-cylindrical, and thick, the pedal muscles strong, the anterior adductor 

 very thin, marginal, and sometimes almost obsolete. 



The transitions from the one to the other of these three genera are, as regards 

 organisation of the animal and the shape of the shell, so manifold and marked that 

 it seems most correct to keep all three within one sub-family. As to the propriety 

 of distinguishing the three genera, which are by some couchologists united imder 



