MANUAL OF THE MOLLUSCA. 



smootli, and ornameiited posteriorly, as also tlie free portion 

 of the right valve, by concentric plications or radiating ribs. 

 Hinge area triangular, transverse, striated in the same 

 direction, edentulous, sometimes produced in tbe middle line ; 

 ligamental furrow median, longitudinal, straight, rather nar- 

 row. Muscular scar near the posterior margin; pallial line 

 wanting. Externally the shells of this genus resemble those 

 of Hinnites and Ostrea. 



Fossil, 5 species. Lower — Upper Lias. France, Germany, 

 Great Britain. 



Family II. — Avicttlidje.* 



Sub-family 1. — PiEEiisrEiisr^. Cartilage contained in a 

 series of linear furrows nearly parallel to the cardinal margin ; 

 hinge-margin broad, flat; anterior muscular scar moderately 

 develojDed and deep. Extinct. 



Genus : — PTEEnsrEA (probably includes the Silurian and 

 Devonian species referred to Avicula). 



Sub-genus : — Eojpteria, Billings ; E, Typica, L. Sil. New- 

 foundland. Yalves equally convex, hinge with an external (P) 

 ligament. 



MoNOPTEEiA, Meek, 1865. 



Type, GervilHa longispina, Cox. Coal Measures. Kentucky. 

 Hinge edentulous ; anterior muscular scar faint, as in Avicula. 



Myalina (see p. 421). Ambonychia (p. 417), (?) AcTmo- 

 DESMA, and Pteeopeeista (p. 416). A. Casei {Megapteria, Meek), 

 Lower Silurian, Indiana, differs from the typical forms of the 

 genus in the great development of its posterior wing. 



Sub-family 2. — Aviculi]^^. Cartilage pit single, defined ; 

 anterior muscular scar very small. 



Genera : — Avicula [Maleagrina, JfaZZews), Vulsella (p. 416) ; 

 Aucella (p. 416), MoNOTis (p. 417), Halobia (p. 417), Posroo- 

 NOMYA (?) Caediola (p. 417); Eueydesma (p. 417). 



Sub -family 3. — PEEis'iiDiE. Cartilage contained in a series 

 of transverse furrows. Anterior muscular scar generally very 

 small. 



Genera :— Peei^a (p. 418), Ceeij^atula (p. 418), Hypoteema 

 (p. 418), Geeyillia (p. 418), Bakewellia (p. 418), and Inoce- 

 RAMUS (p. 419), and the following additional genera : — 



* Seep. 4] 6. 



66 



