1357 



Cyrachaa Leach, Moll. Gt. Brit., p. 314, 1852, Lucina spinifera Montagu ; Ann. Mag. Nat. 



Hist., xx., p. 272, 1847 (name only). 



Ortygia (sp.) Brown, 111. Conch. Gt. Brit, pi. xix., figs. 14-15, 1827. 

 Myrteopsis Sacco, Terz. Piem. Lig., xxix., p. 96, 1901 ; M. taurolcevis Sacco, loc. cit. 



Shell elongate-oval or subrectangular, moderately convex or compressed, 

 dorsal areas obsolete, the sculpture of the disk chiefly concentric and lamellar ; 

 the sculpture less pronounced in the middle of the disk and frequently exhibit- 

 ing a serrate appearance when the lamellae cross the bounding carina of lunule 

 or escutcheon; internally with the left laterals usually obsolete and only one 

 right cardinal tooth ; cardinals entire ; ligament and resilium deep-set but not 

 internal ; anterior adductor scar lucinoid but rather short ; inner margins en- 

 tire. 



This group is paralleled in Phacoides by several others which want the an- 

 terior right cardinal in the adult, but in Myrtaa the single right cardinal seems 

 to be normal, while in the subdivisions of Phacoides its absence is due to de- 

 generation during the growth of the individual or to the dynamic results of the 

 inthrusting of the lunule, which occupies the space where the anterior cardinal 

 would otherwise develop. 



A subgenus of Myrtcea, under the name of Myrteopsis, has been proposed, 

 imperfectly described and unrecognizably figured by Sacco. Not having any 

 sufficient data in relation to the hinge, I place it as a section of Myrtaa, follow- 

 ing the author who proposed it, but accept no responsibility for this arrange- 

 ment. No authority and no reference is given by Sacco in his " Memoir" for 

 the type, which is not figured or described in that work, but he figures very 

 obscurely another species, M. magnotanrina Sacco (loc. cit., p. 96, pi. xxi., figs. 

 32-36) from the Italian tertiaries. 



Three sections may be discriminated : 

 Myrtcsa s. s. 



Sculpture wholly concentric. 



fMyrteopsis Sacco. 

 Larger ; teeth feeble or obsolete, surface smooth. 



Eulopia Dall, 1901. 



Shell with radial vermiculate sculpture between concentric lamellae ; usually 

 of small size. Type Lucina sagrinata Dall, Proc. U. S. Nat. Mus., xii., p. 263, 

 pi. xiv., fig. n, 1887. 



The radial sculpture of the species of this section recalls that of the early 

 representatives of the genus Lucina s. s., in the Oligocene and Eocene, which 

 the later species of Lucina appear to have lost. 



