370 ORErACEOUS PELECYPODA 



There appear to be two or three distinct sections represented in the genus. The 

 one, of the same type as Lith, lithopJiagus, always, I believe, excavates hard sub- 

 stances from the earliest stage of life, and its existence depends upon the presence 

 of solid material, rock or coral, or hard soil. This group is distinguished by a 

 strictly cylindrical form and very small adpressed beaks. Lith. Gruneri is the 

 extreme type of this group, having the beaks somewhat flattened and rather 

 removed from the anterior end. 



11a. The second group is represented by species like Lith. splendida\ Dunk., 

 for which Morch suggested the name Botula (1852 ?). The species are oblong, of 

 thin structure, with flattened, broad, sub-terminal incurved beaks; the cuticule is 

 at the posterior side often produced into long hairy filaments. On the whole, these 

 species barely differ from Modiola, except by their habitat. They attach themselves 

 when young with a byssus in cavities and crevices of rocks and coral. I found them 

 also in sponges and starfishes, but I am not certain whether they really excavate 

 solid rock in the same manner as the true Lithodomi do. It is, therefore, doubtful 

 whether they should be entirely separated from Modiola ; they perhaps rather form 

 a sub-genus of this genus than of Lithodomus. 



116. A third sub-genus is recorded by H. and A. Adams under the name of 

 Leiosolenus, Carpenter, with the only species Lith. spatiosa, in which the animal is 

 said to excavate its burrow by the aperture being prolonged into a kind of a tube. 

 I am not acquainted with the species. 



Species of Lithodomus (as restricted) occur from the lower mesozoic rocks up- 

 wards, but they are not numerous, as may naturally be expected from the difficulty 

 accompanying their discovery. Eossil forms of the type JBotula are not separable 

 from Modiola, except perhaps by the more distinctly flattened and incurved beaks, 

 which are very nearly terminal. 



12. Modiola,^ Lamarck, 1799, fPerna, Adanson, 1756, and Volsella, 

 Scopoli, 1777, ex-parte). Oblong, inequilateral, with the beaks sub-anterior, in- 

 curved, posteriorly more or less produced and expanded, of thin structure ; surface 

 smooth, or sometimes partially radiately striated, and towards the margin more or 

 less corrugate; hinge generally edentulous, sometimes with a few sub-obsolete 

 teeth ; ligament supported by thin fulcra, sub-internal ; muscular scars very un- 

 equal, the anterior much smaller than the posterior, and both not very distinct, 

 as is also the pallial impression ; type, Modiola modiolus, Linn. 



Species of Modiola occur from the oldest deposits upwards. They were already 

 very numerous during Jurassic and cretaceous formations, and the species during 

 the latter are almost quite as numerous as those known recent. 



H. and A. Adams quote two sub-genera, Brachydontes^ Swains., and Adula, 

 H. and A. Adams. 



* It seems quite unnecessary to replace Lamarck's name by either of tlie two older names, Terna and Volsella, 

 for none of them had been used in any restricted sense before the introduction of the name Modiola ; both included 

 a number of various genera. 



