PHYLOGENY OF THE PELECYPODA. 371 



inciirreiit siphon as in later periods is the longer of the two and the two tubes are widely 

 divergi'nt as in the figure. The excurrent sij)hon is not retracted and extended pei-iodi- 

 callj as described in Mya, PI. xxx, fig. 13, and no tentacles surroimd the base of either 

 siphon tube in Petricola. The distal tips of the tubes in Petricola, fig. 6, are fringed by 

 by a few simple unbranched tentacles. 



An older specimen of Petricola p7ioladifo7-mis than that just considered is shown in 

 PI. xxx, fiig. 7. The umbos are too ai'cuately incurved to show the prodissoconch if 

 it were existent, and as a fact, it is not, Ijeing eroded away before this period is reached. 

 The younger portion as indicated by the lines of growth is oval and marked by coarse 

 concentric lines. A distinct stage is indicated in the oval portion of this specimen and 

 similar stages are common, but variable in individuals. The umbos are progressively 

 pushed anteriorly during the younger gi'owth. Rugose plications originate suddenly, the 

 earliest stages having none. From the period of the introduction of plications, the shell 

 rapidly assumes features characteristic of the adult, diftering principally in not being so 

 elongate, differing also of course in that the rugose plications of the adult are produced 

 nearly to what then becomes the umbo of the shell. The siphon tubes of Petricola of 

 the age of fig. 7, are long and are capable of being extended to a length considerably 

 greater than the total length of the shell. They are sepai-ate throughout their length 

 beyond the shell borders as in the younger specimen, fig. 6. In the adult, on the con- 

 trary, Verrill says they are united for about a quarter of their length. In specimens of 

 the age of fig. 7, the branchial siphon, PL xxx, fig. 8, is surrounded at its tip by tenta- 

 cles which branch laterally, intermediate with which are some simple unln-anched tenta- 

 cles. The dorsal excurrent siphon of the same and of similar specimens on the contrary 

 has no branching tentacles, all being simple as in fig. 6. This condition of the orifices 

 is similar to that of the adidt described b}^ Verrill, excepting that he says some bi-anch- 

 ing tentacles exist at the orifice of the excurrent siphon though fewer than at the oiifice 

 of the branchial siphon. 



Ve7ius mercenaria, L. I seciu-ed a number of young shells of this species in the drain- 

 pipe traps at Buzzards Bay. The youngest specimen, PL xxx, fig. 9, has a sharply 

 defined prodissoconch which is yellowish-white, covered with a delicate ei)idermis and 

 bears fine concentric lines of growth. It is separated from the succeeding dissoconch 

 growth by the changes then introduced in the marking of the shell, angles of curvature 

 and color, which though covered with a thin epidermis is nearly chalk-white. The first 

 stages of the dissoconch are marked by the series of concentric ridges and furrows char- 

 acteristic of young shells of this species, but which are wanting in the adult. This 

 feature is shown at the umbos of Dr. Gould's figure of the species. Kidges and furrows 

 like those found in young Venus mercenaria are characteristic of the adult in some sjic- 

 cies of the genus, as Venus rugosa, Chem., of the "West Indies. The same feature is shown 

 in a high degree in the closely allied genus Cliione, and its occurrence in young Venus 

 mercenaria is evidently of genetic significance. In its earliest growth, PL xxx, fig. 9, 

 the dissoconch shows only faint traces of a lunule (which was lui fortunately over- 

 looked when the figure was drawn) but it is a marked feature in the older specimen, 

 PL xxx, fig. 10, and is characteristic of the adult. 



Tottenia gemma, Perkins. A few young specimens were collected in August at Buz- 



