181 



OX THE DISCOA^ERY OF A RECENT SRECIES OF ARCOPERNA. 



By Ralph Tate, 

 Professor of Natural History in the University of Adelaide. 



Mead Uth December, 1896. 



TnE genus Arcoperna was established by Conrad in 1865 (American 

 Journal of Conchology, vol. i, p. 140) for an Eocene fossil [A. Jilosa) 

 of the Mississippi basin. The definition of the genus is unsatisfactory, 

 whilst its classidcatory position is only implied as related to Ifodiola, 

 the author considering M. radiolafa, Beshayes, of the Parisian Eocene, 

 as congeneric ; however, in his " Catalogue of the Eocene Testacea of 

 the United States," op. cit., p. 10, Arcoperna Jilosa is listed iu the 

 family Mytilida3. 



The generic name is evidently compounded of Arra and Perna, the 

 latter being Adanson's name — the equivalent of Modiola of Lamarck. 



Tryon, in his " Systematic Conchology," vol. iii, p. 264 (1884), places 

 the genus next to Modiolaria in the subfamily Crenellinas, and, for 

 differential characters, relies upon the well-marked and comparatively 

 large anterior adductor scar and the fine radiating striation. The 

 foi-mer character seems to me to be valueless for generic diagnosis. 



Cossmann (Cat. ill. Coq. foss. Eocene Paris, p. 161, 1887) redefines 

 the genus, places it in juxtaposition to Modiolaria, and refers to 

 it seven species and a doubtful eighth, which had previously been 

 described by Deshayes and others under Modiola ; one of these occurs 

 also in the Hampshire basin, whilst an additional species is indicated 

 in the Eocene of Belgium. 



The receipt of recent specimens of a species of Arcoperna from 

 Mr. May, dredged off the east coast of Tasmania, came to me as 

 a great surprise, and it is only after confirmation of my determination 

 by my friend M. Cossmann that I venture to make this announcement 

 of a living species of a genus, hitherto only known as belonging to the 

 Eocene of the Mississippi, Paris, Belgium, and Hampshire basins. 

 A re-examination of the Modiolaria-\\kQ shells occurring in the 

 Tertiaries of Australia has resulted in the reference of an undescribed 

 species fi'om the Eocene of the lliver Murray cliffs in South Australia 

 to Arcoperna, but so far the wide hiatus, which separates the recent 

 and fossil species, remains uncontracted. 



Arcoperna, as known in a fossil state, may be briefly described as 

 a Modiolaria with continuous radial ornamentation, not internipted 

 by a wide mesial, non-radiated area as in that genus. The living- 

 examples, however, fui'nish adtlitional differential characters, namely, 

 the vitreous lustre of the interior (although in dead shells a thin nacre 

 is present, probably the result of disintegration) and the absence of 

 a distinct periostraciim. 



VOL. II. — JULY, 1897. 13 



