474 CRETACEOUS PELECYPODA 



margin or dental processes of the flat valve corresponding to one or two ligamental 

 furrows of the other valve; muscular scar single, pedal impressions distinctly 

 double or single, sometimes apparently absent. 



All the Anomiid^ have a comparatively thin shell of a subnacreous, lamellar, 

 and often punctated structure, resembling that of the Brachiopoda. 



Having separated the Flacunidm as a distinct family, I include in the 

 Anojuiidje all those forms which have one valve flatter than the other and usually 

 perforated. Only in one fossil genus fDijjloscJdzaJ are both the valves perforated, 

 luit there are again some other genera which show no perforation at alL From 

 the study of the shell of Carolia, which apparently has when young a distinct 

 foramen, and when adult only an internal groove indicating it, it appears probable 

 that most of the genera have in their young state a small perforation, or emar- 

 gination, in the lower valve. 



The fossil representants of this family are not numerous, the shells being too 

 thin to be well preserved under ordinary circumstances. Some peculiar forms 

 are found already in pala30zoic beds, but the family attains a geological importance 

 only with the appearance of Placunoims in the Jurassic rocks. Erom cretaceous 

 rocks several peculiar genera have been described, but true Anomice also occur. 

 The species from tertiary beds are more satisfactorily identified with recent genera, 

 although, as a rule, the determination of the fossil species is very difficult, because 

 the upper convex valve, (which is the only one generally found preserved), is exter- 

 nally in all the genera almost quite similar, and the opportunities of examining the 

 internal form of the ligamental pits are far from numerous, and still fewer those of 

 finding the smaller valve attached to the larger one. 



According to geological history the genera arrange themselves in the following 

 order : Limanomki, ( Devon. ) ; Anomicmella, ( Carb. ) ; Flacunopsw ( Jur. ) ; Para- 

 nomia, the ]}eculiav Ci/closfreon and Dijoloschiza, (Cret. ); Carolia, (Eoc), Podo- 

 desmus, Placnuanomia, Anomia and JEwcjnia (recent). Anom'ia is recorded already 

 from the jurassics and continues through all the successive formations. 



The recent species may be estimated at about 50, but the amount of variation 

 of certain species appears to be very great, and some authors admit barely more 

 than one-half of the number mentioned. They possess a general geograpliical dis- 

 tribution and range from the littoral zone to several hundred fathoms. 



As regards the zoological affinities of the present to the previous family the 

 genera can be arranged as follows : — 



1. Placuuopsis, Morr. and Lycett, 1833, (Moll. Great Ool., pt. ii, p. 5). 

 Sub-orbicular, generally somewhat irregular, inequivalve ; larger valve convex, 

 with small sub-marginal, sub-median beak, and mostly ornamented with radiating 

 ribs or strite; smaller A'alve flat, free, or attached to foreign objects; hinge toothless, 

 Avith a small cartilage jiit in each valve ; muscular scar large, sub-elliptical, sub- 

 central. Type, P. Jurensis, Room. All the species as yet known are from Jurassic 

 deposits. The inequality of the valves and absence of hinge-teeth, or any other 

 internal processes, separate this genus from the next one, but it is not certain 



