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NEW YORK STATE MUSEUM 



the Protoconchae depart toward the monomyarians, the Staurogoniacea 

 (unknown ; to be sought in the Paleozoic), the Opisthogoniacea (Avicula), 

 the Symptogoniacea (Pecten). It is not practicable in this place to enter 



into a further statement of these determinations ; 

 the reader must look to the original article or its 

 English abstract. 



Referring again to our figures, we find that, if 

 the adult shell of Lunulicardium be construed and 

 oriented as a dimyarian, then the angle between 

 o-a and w (crescence-line) is less than 90 and con- 

 forms to this angle in the extreme dimyarian line. 

 But in such orientation we unavoidably reverse the 

 extremities of the oro-anal axis, the anal extremity, a, 

 appearing on the byssal side, which we know to be 

 a condition not existing in the lamellibranchs. The 

 inference therefore, that Lunulicardium has little to 

 do with the dimyarians, is supported by empirical 

 observation of a single posterior adductor or a 

 median approximation and fusion of the two 

 adductors. Hence, orienting the shell as monomy- 

 arian, 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 



toconchae. In comparing this with ° * 



Noetiin/s figures it is necessary to ^ a ke this as excellent confirmatory evidence of the 



bear in mind that here the exte- 

 rior of the vaive is represented monomyarian affinity of the genus. In following 



while in that the interiors are 



given. This will account for the the torsion of the shell backward trom the adult 



apparent reversal of direction in «• • i- i /» 1 i ■ 



the two. condition to the prodissoconch we find that it passes 



through the angles necessary to bring it with precision to the condition 

 of the Protoconchae, and the relations of the axes in the latter also cor- 

 respond with those of the prodissoconch. I believe therefore that this 

 larval shell is an actual representative of the Protoconchae condition, 



Fig. 7 Diagram giving outline 

 of adult Lunulicardium cly- 

 m e n i a e with larval shell attached: 

 o-a, oro-anal axis in adult shell; 

 o-a, the same in young shell; w, m, 

 crescence line in the two. The 

 lower arc indicates the very large 

 critical angle for the adult stage, 

 the upper arc, the critical angle 

 for the young stage. The former 

 is normal to a position between 

 the Opisthogoniacea and Sympto- 

 goniacea, the latter in agreement 

 with the critical angle of the Pro- 



