INVERTEBRATE PALAEONTOLOGY. 39 



the longer (vertical) axis of the valves; posterior dorsal region some- 

 times compressed and subalate; anterior side straight or broadly sinu- 

 ous below the beaks, which are anterior and little oblique, excepting 

 at the points, the left one of which is nearly always more prominent, 

 and incurved than the other. — Type as already stated. 



b. Shell with posterior dorsal region compressed and extended, 

 so as to form a wide alation, thus imparting a more or less subtrigonal 

 outline to the valves, and giving greater length to the hinge. — I. alatas, 

 Goldfuss. 



2. mytiloides, Brongniart. 



Shell obliquely ovate, elongate, or somewhat mytiliform, com- 

 pressed, very nearly or quite equivalve ; hinge-margin generally very 

 short, and oblique to the longer axis of the valves; .posterior dorsal 

 margin sometimes a little compressed and alate. Surface with 

 irregular undulations and striae. — /. problematicus, Schloth. 



3. catillus (Brongniart?), Chenu. 



a. Shell transversely oval or oblong, equivalve or subequivalve ; 

 beaks depressed, nearly or quite equal, and more or less anterior, 

 but not terminal ; hinge long, and ranging parallel to the longer 

 (transverse) diameter of the valves. Surface concentrically undulated 

 and striate. — /. Lamarckii, Brong. (as figured by d'Orbigny in Palseont. 

 Fr., Ill, pi. 412). 



b. Shell usually more compressed and proportionally shorter, 

 or broad truncato-suborbicular in outline, with the depressed beaks 

 at the anterior end of the hinge. Surface undulations, generally 

 irregular. — /. latus, Mantell. 



4. actinoceramus, Meek. 



Shell obliquely or nearly vertically subovate, higher than the 

 transverse diameter, gibbous; hinge very short, rather oblique; beaks 

 pointed, terminal, that of the left valve more prominent than the 

 other, and curved forward and inward. Surface with radiating plica- 

 tions or costac, and concentric striae. — I. sulcatus, Parkinson. 



