244 REPORT ON ALBATROSS MOLLUSCA DALL. 



to and possibly homologous with an original ossiculum has (that view- 

 being taken) revolved around the cartilage, taken its place outside of 

 the axis of motion of the hinge, and instead of keeping the valves from 

 crushing each other by checking the closing stress of the adductors as 

 in Vertivordia or Bushia, and other Anatinacea, it accomplishes the same 

 end by locking over the reflected edges of the shell on the dorsal sur- 

 face, acting like the anterior adductors on the short instead of the long 

 arm of the lever, and, as before, in a sense opposed to the action of the 

 adductors. Though greatly specialized and modified, this appendage 

 retaina something of the butterfly shape of a broad ossiculum. 



An appendage, sometimes called the styliform process or apophysis, 

 with its proximal end attached in the hollow of the beaks, has been 

 homologized by Deshayes with the cardinal teeth. In Pholas costata 

 it supports the posterior oral palpus, which is very massive, and some 

 of the internal viscera. If one of the umbonal lamina?- of CaUocar- 

 dia were detached from its connection with the cardinal margin and 

 allowed to project into the cavity of the valve it would somewhat re- 

 semble the apophysis of Pholas. But on this view I am at a loss to 

 explain the present connections of this process, about the development 

 of which little or nothing is known. How a cardinal tooth should 

 come to be situated inside the mass of the body would seem to be hard 

 to explain. 



The environment of the Pholads is of a very special character, and the 

 modifications of the organization march with the peculiar circumstances 

 under which it exists. To enter into their mutual reactions would take 

 much space and obscure the more general questions to which these 

 remarks are addressed. 



It may be added that in this order, as well as the others, the partic- 

 ular constituency of each of the suborders, even the number and scope 

 of the families, must be regarded as tinged with uncertainty from the 

 magnitude of our ignorance. To properly ascertain and correlate the 

 data in regard to the different genera and the families of which the> 

 are the members is a labor worthy of devotion, but which will yet 

 require a large amount of original research. 



In the Prionodesmacea the Nueulacea represent an archaic type in 

 many of their features. So far as the hinge is concerned Area (now 

 and related species) is perhaps the most fully and typically developed 

 instance of Prionodont dentition. The Naiades declare in Spatha and 

 Iridina their Prionodont origin, traces of which are to be seen in the 

 transverse striatiou of the teeth of many species of Unio, even when 

 lateral teeth have become well developed and pre-eminent. The same 

 is true of Trigonia, which has many points in common with the Naiades 

 and may perhaps be the descendant of a common ancestry. To the 

 latter immediately Mulleria bears such a relation in its adult state as do 

 the Monomyarian Pecten and Ostrea to the rest of the Prionodesmacea 

 as a whole. The Prionodont character of the Mytilacea will not be 



