PHYLOGENY OF THE PELECYPODA. 359 



with active extensible tentacles which are in constant motion. The tentacles of the yonng 

 are pigmented with black in scattered spots and bands of color which are arranged at 

 right angles to the longer axis of the tentacles. I did not observe radial muscles in the 

 mantle of Anomia similar to those described In Ostrea and Pecten, and if they exist as 

 they jH'obalily do, they are inconspicuous. 



Xlir. Anomia. The shell. 



Professor Morse published a brief account of some extremely young shells of Anomia 

 in which he showed that an early, probably ''dimyarian" shell precedes the characteris- 

 tic Anomian form of shell. He also showed the initial stages of the byssal notch. The 

 specimens studied, judging from his description and a comparison with my own observ- 

 ations, were the young of was Aiiomia aculeata, Gmelin. 



In AnomiM glabra an ovally rounded, nearly equivalvular prodissoconch exists at the 

 umbos of perfectly preserved young specimens, PI. xxix, figs. 4-6. The prodissoconch 

 in this sj^ecies is opaque, of homogeneous structure, bearing fine concentric lines of 

 growth, and the umbos which are neai-ly central are directed slightly toward the poste- 

 rior end of the shell.' In this position of the umbos my observations differ from those 

 of Professor Morse as he says the umbos are nearer the anterior margin of the shell. 

 The left prodissoconch valve of Anomia glabra has no indication of a byssal notch but 

 the right valve has a notch on its ventral border as shown by the lines of growth in PI. 

 XXIX, fig. 6. It is therefore seen that at this extremely early period Anomia is already 

 capable of forming a byssal attachment, and crawled on the right side with the foot ex- 

 tended at this area. The shell evidently lay on one side as observed by Morse. A 

 byssal notch in the prodissoconch is a point in which Anomia differs from the byssated 

 Pelecypods which have been so far discussed. In Perna, Avicula, Pecten and Spondy- 

 lus, PL xxvn and xxviii, a byssal notch is indicated in the first stages of the dissoconch 

 growth; but no trace of a notch exists in the prodissoconch. 



The dissoconch growth of the left valve o? Anomia glabra in its early stages, PI. xxix, 

 fig. 4, is at first nearly round, but slightly larger on the ventral side. It is thin, translucent, 

 has a highly polished surface and bears numerous, delicate lines of growth, but shows no 

 prismatic structure. An older specimen of a left valve, PI. xxix, fig. 5, is less rounded, its 

 faster ventral growth having produced a bulging in this direction which seems to be char- 

 acteristic of specimens of this age. Whereas in fig. 4, the prodissoconch is on the anterior 

 border of the valve and projects slightly beyond it, in fig. 5, shell growth is taking place 

 in front of the prodissoconch, which is the primitive umbo of the valve, and in succeeding 

 growth this feature is carried still farther as seen in fig. 8, so that in the adult the ini- 

 tial mnbo is pushed in quite far from its original position on the anterior border of the 

 valves. As growth continues from the stage fig. 5, the shell becomes more and moi*e 

 rounded, rapidly assuming the form characteristic of the adult. The young is so highly 

 polished that the shell well deserves the popular name silver-shell which it bears. 



The right valve at a very early peiiod, PI. xxix, fig. 6, difiiers much from the adult 



'The umbos are directed posteriorly so sliijhtly in Ano- Pecten, etc., in whiclithe umbos of tlie prodissoconch are 



mla that it is not a marked feature as in Ostrea, Perna, more inclined toward the posterior region of the shell. 



