TESTICARDINES : INTERNAL CHARACTERS 



503 



scar (jj) ill the ventral valve has before it a pair of " accessory 

 clivaricator " scars {a.d) flanked hj a pair of " ventral adjuster " 

 (r.«) and a pair of " divaricator " impressions (r/), between which 

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

 posterior pairs of occlusor scars (a.a, a.p) are visiljle. 



The minute structure of the calcareous shell of the Testi- 

 cardines is of flattened fibrous prisms inclined at a very acute 



Fig. 334. — Leptaenn rhomboidaUs. (Silurian.) 

 A, I'].\ternal view of ventral valve. B, In- 

 terior of ventral valve : a, occlu.sor scars ; 

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

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

 dorsal valve : a.a, anterior occlusor scars ; 

 2). a, iJosterior oc- 

 s ,_ '^'^ y& Cci clusor scars; c.a, 



hinge area ; c.}), car- 

 dinal process ; d, 

 cliilidium ; s, dental 

 sockets ; v.s, 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 (Froductus) 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 .-^npport.s. 



(For muscles see Fig. 322.) 



Range. — Lower Cambrian to Recent. 



Principal Genera. — LiiKjula, Lhujnldla, Lin(jule])is. 



