NO. 1 SOOT-RYEN : THE FAMILY MYTILIDAE 57 



having one tooth. In fact, Modiolus species usually have one toothlike 

 projection where the anterior margin stops below the ligament. This pro- 

 jection is easily seen in most of the specimens of every size, not only in 

 young ones, as mentioned by Dall, Bartsch, and Rehder (1938) ; and the 

 presence of this projection proves that Scopoli was a careful observer. 

 Though the use of Folsella now is generally accepted, the International 

 Commission on Zoological Nomenclature in Opinion 325, issued January 

 7th, 1955, has made a suspension of the rules and accepted Modiolus 

 Lamarck 1799 as a nornen conservandum according to a motion by J. L. 

 Baily, Jr. This change came after this paper was in print and the neces- 

 sary corrections had to be made during the proof reading. 



The typical Modiolus has a smooth, elongate shell with slightly curved 

 ligamental margin, a curved dorsal margin without a pronounced dorsal 

 angle, and the umbones placed slightly behind a rounded anterior margin 

 which is bent slightly outward. The anterior adductor is elongate and 

 placed ventrally, the anterior retractor leaves a rounded scar in or behind 

 the umbonal cavity, the posterior adductor is placed in the upper half 

 of the shell and is continuous with the long and narrow bundle of pos- 

 terior retractors. The periostracum is heavy and furnished with hairlike 

 protuberances, at least on the dorsal and posterior part. On the anterior 

 margin, a lunule is visible by the dull periostracum, especially in younger 

 specimens. The resilial ridge is compact, and the nymphae, usually strong, 

 are rather distant from the dorsal margin, making the ligamental area 

 broad. The dorsal siphonal opening is oval with smooth margins and 

 does not form a siphon which can be extended ; the mantle of the bran- 

 chial opening is thin or thickened, but always without tentacles or papil- 

 lae. 



Unfortunately a multitude of species with the outline of Modiolus but 

 with radiating sculpture, different placement of the muscles, or differ- 

 ences in the anatomy, have been referred to this genus. Many of these 

 species show affinities to other groups, e. g., to the Musculus group 

 (Lioberusjj and some should be placed in new genera. Before a more 

 correct grouping of the species can be completed, the anatomical and 

 conchological characters must be studied carefully. Until then, the 

 species which are doubtfully allocated to this genus should be cited as 

 "Modiolus" sp. 



The Modiolus type can be traced back in time to the first origin of the 

 family Mytilidae and certainly represents the oldest element of this 

 family, or perhaps forms the continuation of the Modiolopsidae. The 

 flatter species of Volsellina Newell seem to be closely related to some 

 recent Modiolus-\\\it species. 



