74 THE NyXUTlLUS. 



buiTOwei's retire is a conniaratively tliiii coating over a stony or 

 rocky layer wiiich they cannot pierce, tiie tendency In Panopea, 

 Mya, etc., is for relatively short and broad shells, with shorter 

 siphons, to survive ; which naturally have a wider, shorter, and more 

 rounded pallial sinus, and shorter and more incurved nymphs. I 

 believe the influence ot" environment is direct and not selective ; at 

 all events the association of situs and specimens so characterized is, 

 as far as I have been able to determine, quite uniform, whetlier 

 selective or not." 



Wliile liviiio; at St. Augustine, Florida (1880-87), I was fortu- 

 nate- in finding in the harbor, on a sand bar near " Marsh Island," 

 a specimen of Panopea bitruncata with valves intact, and from which 

 the animal had apparently just been removed. This s{)ecimen which 

 is shown on plate IV, represents a nearly normal shell (reduced ;ilu)iit 

 one-fifth) with the lines of growth but slightly interrupted antei-iorly. 

 It had probably grown under favorable conditions in the adjacent 

 sandy-mud bottom. The shell measures 133 mm. (5.25 inches) in 

 length, with a width of 80 mm. On the ocean beach I also found 

 several single valves ; these were proportionately shorter, and widi-r, 

 giving them a more truncated appearance. In my list of the slidls 

 of St. Augustine, P^'lorida, in this Journal, Volume IV, page 4, I re- 

 ferred these to Glycimeris hitruncala Conr., while in naming the one 

 from the harbor, I followed Dr. Ball's catalogue of the shell-bearing 

 moUusks of the southeastern O;oast of the United States (Bull. 37, 

 U. S. Nat. Mus.) and called it G. rejlexa. Say. 



Since Dr. Dall's review of the species (Trans. Wagner Free Inst. 

 Sci. Vol. Ill, pi. 4, p. 831), I have made a careful study of the tyjie 

 of P. bitruncata in connection with all recent specimens obtainalde 

 and find no greater variation than exists in the Pliocene specimens of 

 Florida. The type of P. bitruncata is an injured specimen ; the 

 upper or dorsal portion of the posterior end being broken aw.iy, 

 gives the shell a very oblique truncation, while the lower portion of 

 the anterior has been frequently arrested in its development, the 

 lines of growth being interrupted and crowded together, gives tluit 

 end also a very oblique outline. The umbonal and younger {)ortioii 

 of the shells are alike in all the specimens I have e.xamined. 



Uniting the recent and Pliocene forms, and adopting the oldest 

 name will make the synonomy stand as follows: 



