REPORT OP THE STATE PALEONTOLOGIST 1902 1233 



ing in the lamellibranchs. The inference therefore that Lunuli- 

 cardium has little to do with the Dimyarians is supported 

 by empirical evidence of a single adductor or median approxima- 

 tion or fusion of two adductors. Hence orienting the shell as 

 Monomyarian posited between Avicula (Opisthogoniacea) and 

 Pecten (Symptogoniacea), on the basis of its muscular and byssal 

 structure, we find that the critical angle is exact for the position 

 taken. We take this as excellent confirmatory evidence of the 

 Monomyarian affinity of the genus. 



Fig. 6 L un ulicardi um miilleri. Fig. 7 L . hemic ardioid es with sin- 



Internal cast showing approximate muscle gle large adductor scar 

 scars 



In following the torsion backward from the adult condition to 

 the prodissoconch we find that it passes through the angles neces- 

 sary to bring it with precision to the condition of the Protocon- 

 chae and the relations of the axis in the latter also correspond 

 with those of the prodissoconch 1 . I believe therefore that the 

 prodissoconch stage is an actual representative of the Protocon- 

 chae condition and that the stages of torsion of the Lunulicar- 

 dium shell in subsequent growth are indicative of its phylogenic 

 stages of progress toward the Monomyarian stock. This con- 

 clusion is based wholly on external characters and the fact of 

 actual observed torsion in the shell from youth to adulthood. 

 The Protoconchae being theoretically the primitive lamellibranch 

 condition, we may never come to know it except in this or a 

 similar manifestation. The supposed position of the pallial line 

 in this group, just within the hinge, is a feature on which the 

 nature of our material permits no observation. 



1 The reader will note that we are here dealing with the exterior of these shells. 

 Noetling's figures are based on interiors ; hence the relations of the latter and 

 our figures are reversed. 



