SUBMYTILACEA. 



725 



Fig. 601 



Interior of the 



In the genus Astarte (fig. 601) the shell is thick, and usually 

 concentrically furrowed, subtrigonal, rounded, or oval in form, 

 inequilateral, and usually with a well- 

 marked "lunule." The ligament is ex- 

 ternal, and there are two cardinal hinge- 

 teeth in each valve, the front tooth of 

 the right valve being large and thick. 

 The genus Astarte is stated to occur in 

 the Palaeozoic series, from the Silurian 

 onwards ; but the affinities of the supposed 

 ancient representatives of this type are 



uncertain. From the Mesozoic deposits right valve of Astarte detrita 

 numerous species of Astarte have been ob- J urassic - 

 tained, and the genus attained its maximum development at this 

 period. The Tertiary species are fewer in number, and about a 

 score of living species are known. The Carboniferous genus 

 Astartella and the Cyfiricardella (Microdoti) of the 

 Devonian and Carboniferous rocks appear to be 

 related to Astarte. The genus Pac/iydo/nus, of the 

 Devonian rocks of Australia and Tasmania, may, 

 perhaps, also be placed in this family. 



Of the remaining genera of the Astartidce, the 

 only one which need be specially noticed is Opts, 

 of which numerous species are found in the Triassic, 

 Jurassic, and Cretaceous rocks. In this genus the shell (fig. 602) 

 is heart-shaped and keeled, with prominent incurved beaks, a 

 distinct lunule, and a single cardinal tooth in each valve. 



Family 7. Crassatellid.e. — The general characters of the 

 animal in this family are similar to those in the Astartidai. 



Fig. 602. — Inte- 

 rior of the right 

 valve oiOpis lunu- 

 lata. Jurassic. 



Fig. 603. — Crassatella ponderosa. Eocene Tertiary 



The shell also agrees with that of the Astartidce in being equi- 

 valve, massive, and subtrigonal, with a distinct " lunule." The 

 shell is, however, more or less produced posteriorly, and the liga- 

 ment is lodged in an internal groove. The type of this family 

 is Crassatella itself (fig. 603), in which the shell is thick, solid, and 



