TESTICARDINES : INTERNAL CHARACTERS 



503 



scar (jt?) in the ventral valve has before it a pair of " accessory 

 divaricator " scars Qa.ci) flanked by a pair of " ventral adjuster " 

 (y.a) and a pair of '' divaricator " impressions (c?), between which 

 lie the two occlusor scars (ci). In the dorsal valve anterior 

 and posterior pairs of occlusor scars (a. a, a.jt?) are visible. 



The minute structure of the calcareous shell of the Testi- 

 cardines is of flattened fibrous prisms inclined at a very acute 



Fig. 334. — ie/)«aena rhomhoidalis. (Silurian.) 

 A, External view of ventral valve. B, In- 

 terior of ventral valve: a, occlusor scars; 

 d, pseudo-deltidium ; d.v, divaricator scars ; 

 c.a, hinge area; t, teeth. C, Interior of 

 dorsal valve: a.a, anterior occlusor scars; 

 p. a, posterior oc- 

 clusor scars ; c.a, 

 hinge area ; c.p, car- 

 dinal process ; d, 

 chilidium ; s, dental 

 sockets; ?;. 5, vascu- 

 lar sinuses. 



angle to the surfaces. In many forms minute tubes more or 

 less closely arranged pierce through the fibrous shell-substance ; 

 but in some genera QProductus) they do not reach the outer 

 surface (see p. 468). Allied genera, however, differ much in 

 the punctate or impunctate character of the shell. 



Synopsis of Families 



I. ECARDINES 



Family. Lingulidae 



Shell elongated, composed of alternating chitinous and calcareous layers, 

 the latter of which are perforated. Attached by a pedicle passing between 

 apices of valves. 



Arms have no calcified supports. 



(For muscles see Fig. 322.) 



Range. — Lower Cambrian to Recent. 



Principal Genera. — Lingula, Lingulella, Lingulepis. 



