220 PEOCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



consequently referable to the same group as M. exustus, a group -whicli 

 has been separated by Morch. as the subgenus Horniomya. I cannot 

 therefore recognise the subgenus Trichomya as having any value 

 whatever. 



Other species, generally called Mytihis, which I would transfer to 

 Modiula are : M. confusus, Angas, M. ater, Frauenfeld, and M. atratus, 

 Lischke. 



With regard to the subdivisions of Modiola, I cannot, of course, 

 accept the sections and subgenera adopted by Dr. Dall in 1898,^ 

 because he includes those with a crenulated hinge-line, like 

 Brachydontes and Oregariella, which I transfer to another genus. 



A few species of Modiola have smooth, polished shells, and these 

 have been grouped under the name Amygdalum ; it may be convenient 

 to recognise this group as a natural section of the general assemblage, 

 but the character on which it is based does not seem to me of much 

 importance. There are other forms, such as M. mirieulata, Krauss, 

 31. lignea, Reeve, and M. flmida, Dunker, which differ from the type 

 and from one another, and may be separable as subgenera, but the 

 whole assemblage of toothless Mytilidce requires further investigation, 

 not only as regards the shells, but the animals themselves. 



Myrina has been placed by Fischer as a subgenus of Modiola, and 

 may perhaps be so retained, for though its course of hinge-develop- 

 ment, as described by Bernard, is slightly different, it practically 

 becomes a Modiola when adult. Adula is another group which may 

 be placed here, but is certainly a link between Modiula and 

 Lithodomus. 



Lithodonms itself next claims attention, and if I were classifying 

 the Mytilidae for the first time, I should not think it worthy of 

 generic separation, but should regard it as a subgeneric group, 

 obviously derived from Modiola, and modified only in consequence of 

 having taken to excavating holes in hard substances. The longer 

 siphons of the animal and the cylindrical form of the shell are the 

 most obvious points in which Lithodomus differs from Modiola ; and 

 in Botula, as well as Adula, there are certainly connecting links. 



I find, however, from a perusal of Dr. Th. List's excellent monograph ^ 

 of the Mediterranean L. Uthophagus that the muscular system does 

 differ to some extent from that of Modiola. The anterior adductor 

 is relatively stronger, and the posterior smaller, so that the two 

 adductor scars are nearly equal in size ; the posterior retractor byssi 

 forms a small oval scar directly above the adductor, and there is no 

 separate pedal scar, the muscle being rudimentary, and its terminal 

 surface united with that of the retractor byssi. Thus the shells which 

 retain a definite anterior expansion of the shell, with a smooth interior 

 margin, and present a hinge-line entirely destitute of teeth or 

 crenulations, may be grouped in the two genera Modiola and 

 Lithodomus. Their sections and subgenera may be tabulated as below. 



^ Trans. Wagner Free Inst. Sci. Philadelphia, vol. iii, pt. 4, p. 791. 

 2 "Fauna und Flora des Golfes von Neapel " : Zool. Stat, zu Neapel, Mon. 27, 

 Die Mytiliden, von Dr. Th. List, 1902. 



