PHYLOGENY OF THE PELECYPODA. 371 



inciTrrent siphon as in later periods is the longer of the two and the two tnbes are widely 

 divergent as in the figure. The excurrent si[)hon is not retracted and extended periodi- 

 cally as described in Mya, PI. xxx, fig. 13, and no tentacles sui-round 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 unbranehed tentacles. 



An older specimen of Petricola jiliolculiformis than that just considered is shown in 

 Pi. XXX, fig. 7. The umbos are too arcuately incui'ved to show the prodissocouch if 

 it were existent, and as a fact, it is not, being 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 ai'e progressively 

 pnshed anteriorly dni-ing 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, differing principally in not being so 

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

 nearly to what then becomes the nmbo 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 consideral)ly 

 greater than the total length of the shell. They are sei)ai-ate thi-oughout their length 

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

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

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

 cles which branch laterally, intermediate with which are some simple unbranehed 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 adult described by Yerrill, excepting that he says some branch- 

 ing tentacles exist at the orifice of the excun-ent siphon though fewer than at the orifice 

 of the branchial siphon. 



Venus mercenaria, L. I secured a number of young shells of this species in the drain- 

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

 defined prodissocouch which is yellowish-white, covered with a delicate e])idermis and 

 bears fine concentric lines of growth. It is sejiarated from the siicceeding 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. Eidges and fiuTows 

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

 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 Chione, and its occurrence in young Venus 

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

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

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

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



Tottenia yemma, Perkins. A few young specimens wei-e collected in August at Buz- 



