1232 



NEW YORK STATE MUSEUM 



apex of the prodissoconch are distinctly turned away from the 

 byssal cleft or backward. 



With this orientation of the shell the beaks are opisthogyre. 

 The larval shell or prodissoconch lies with its apex or primitive 

 beak directed downward or toward the postlateral extremity of 

 the adult shell, so that the original posterior extremity of the 

 larval shell actually lies at and constitutes the beak of the adult. 

 This relation is shown in the accompanying figures. The shell 

 has in growth actually twisted with reference to the animal and 



the axial line which we have 

 marked o-a has apparently trav- 

 eled through a large angle to 

 reach the corresponding position 

 in the adult, O A. The muscula- 

 ture consisted probably of a 

 single adductor corresponding 

 to M on Noetling's diagram. 

 This expresses itself on the best 

 preserved internal casts as a 

 simple moderately large scar as 

 shown in the figure adjoining of 

 Lun. hemicardioides or 

 it may be an approximation of 

 two central scars as seen in 

 Lun.niulleri. In the former 

 the condition seems to be the 



„- . . ... . Lunulicardium clymeinae wit 



OUtCOme 01 IUSlOn Ot the apprOXl- larval shell attached: o-a, oro-anal axis in 



Fig 5 Diagram giving 



adult 



shell is thus in effect Monomya- 

 rian, though of Dimyarian in- 

 ception. 



If now the shell of Lunuli- 



adult shell; o-a the same in young shell; 

 mate SCarS Of the latter. TllC w » «>. crescence line in the two. The lower 



arc indicates the very large critical angle 

 for the adult stage, the upper arc, the criti- 

 cal angle for the young stage. The for- 

 mer is normal to a position between the 

 Opisthogoniacea and Symptogoniacea, the 

 latter in agreement with the critical angle 

 of the Protoconchae. In comparing this 

 with Noetling's figures it is necessary to 

 bear in mind that here the exterior of the 

 valve is represented while in that the in- 

 Cardium be Construed and Ori- teriora are given. This will account for 



the apparent reversal of direction in the 



ented as a Dimyarian then the two 



angle between O-A and W (crescence-line) is less than 90° 

 and conforms 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, or A, appearing 

 on the bvssal side which we know to be a conditioo not exist- 



