TRANSACTIONS OF WAGNER 

 872 



TERTIARY FAUNA OF FLORIDA 



exceptional adductor muscle sufficient for the purpose, then, in time, other 

 teeth will appear and contribute their quota to the dental armature. 



Specifically, in the case of Mactra, Neumayr (pp. cit., If 4) homologizes, 

 first, the nymphre of Glycimcris (Aldrovandi) and Mya tnrncata, showing their 

 intimate relationship. He does not mention the fact that the ligament in both 

 these forms is composed of an outer, posterior, slightly greenish, ligamentary 

 portion, and an inner, more ferruginous part, set off by a calcareous uncon- 

 solidated layer from the former. Tlfe latter is really an internal "cartilage" or 

 resilium, and ,the scars of insertion of the two parts are readily distinguished 

 on careful examination. In the fresh or alcoholic specimen the distinction 

 of color enables one to recognize the parts at a glance. He proceeds to com- 

 pare Mya with Thracia (phaseolina), and here again the homology is readily 

 recognized. But when he continues by homologizing the cardinal teeth of 

 Eastonia (rugosa) with the marginal ridges of the resiliary pit, and these with 

 the stout rib below the ligamentary groove of Panopea or T/iracia, a halt 

 must be called at once. They are in no respect homologous. The ridge in 

 Panopea is- homologous with the /// of Mactra. In the former it is convex, 

 in the latter concave; in both it is the seat and fulcrum of the resilium, whose 

 very existen.ce has not been recognized because it is, so to speak, wrapped in 

 the ligament in Panopea. It is probable that Neumayr's observations on 

 Mactra were based on fossil specimens, or recent ones which had lost their 

 ligament and resilium in drying. Otherwise some of his remarks would be 

 incomprehensible. 



The error into which he has been led is still more obvious in the contin- 

 uation, where he separates Rangia, as having a typically Heterodont hinge, 

 from Mactra, which he regards as a true Desmodont. I have elsewhere 

 shown that both in its gross anatomy and its hinge Rangia is truly Mactroid, 

 and cannot be separated when young from a young Alii/inia. If one is the 

 other must also be Heterodont. The peculiarities of the sunken ligament 

 and resilium are shared with Mulinia, and the deep pit to which attention was 

 especially called by Neumayr is the dynamic result of the wide separation of 

 the umbones and the persistency of the resilium. He would have found the 

 same extended ligament, uncovered, in Schizodcsma ; and, in fact, in all 

 Mactridce the ligament starts at the beak of the shell or nearly so, and its 

 termini are widely separated or close together according as the beaks are far 

 from or near to each other. 



In Mactra the cardinals represent radiating arms of a bent lamina primi- 



