332 



ROBERT TRACY JACKSON ON THE 



'^^sft — .= 



nla as far as can be ascertained in its partially eroded condition. The byssal sinns of 

 the right valve originates in the initial stage of the dissoconch growth, as in Avicnla. 

 In Pseiidomonotis echinata, sp., Sow., I have been able to observe the condition of the 

 yonng in specimens from the Cornbrash of Wilts, England, No. 10737, Museum of Com- 

 parative Zoology. A well-preserved prodissoconch like that of Avicnla, exists at the 

 umbo of either valve. The nepionic stage of the left valve, which seems to be the more 

 normal, least modified valve in this species, is sub-rhomboidal in form and resembles the 

 same stage of Avicnla, fig. 32, p. 330. In the right, flatti-.h valve, the byssal sinus orig- 

 inates with the inti'oduction of the nepionic stage as in Avicnla, etc. 



In some well-preserved specimens of right valves of Oxjtoma ineqidvalvis, sp. Sow., 

 from the middle Lias, PI. xxvii, fig. 7, I found a well-preserved prodissoconch. As in 

 related genera the umbos of the prodissoconch are directed posteriorly, and the byssal 

 sinus originates Avith the introduction of the nepionic stage on the ventral border of the 

 prodissoconch as shown in the figui-e. 



In specimens of Cassianella from the Triassic, a prodissoconch has been found similar 

 to that of Avicnla. It is worth noting that Cassianella is from the oldest geological 



formation from which I have so far succeeded 

 in finding a Pelecypod with the prodissoconch 

 still intact. Dr. Beecher sent me recently 

 specimens of Cassianella decussata, Miinst, 

 which show the feature mentioned, fig. 35. 

 The prodissoconch is ovate with umbos di- 

 rected posteriorly, and it is sharply defined 

 from the succeeding dissoconch, as in Avicnla 

 and allied genera. The nepionic stage of 

 Cassianella is similar to the same stage of 

 Avicula (compare tip of valve of fig. 35, with the whole valve of fig. 32, p. 330) . The 

 prodissoconch has also been observed in Cassianella gr-yplieata, Miinst, from the Triassic. 

 In specimens of Vtdsella ricgosa, Lam., kindly given me by Professor Verrill, I have 

 found stages of interest. At the umbos of a J^oung individ- 

 ual, fig. 36, a well-defined prodissoconch exists similar to 

 that of Avicula. The prodissoconch is oval with umbos di- 

 rected posteriorly. It is composed of homogeneous lime, 

 and bears fine concentric lines of growth. The nepionic 

 stage of the dissoconch, as in related genera, is sharply de- 

 fined from the i^rodissoconch by its changed form and histo- 

 logical structure. The shell of the nepionic and later stages 

 is composed on the outside of prismatic cellular tissue, with 

 nacreous tissue in the deeper seated parts. The form of the 

 nepionic stage is sub-rhomboidal, bearing a certain resem- 

 blance to the same stage of Avicula, fig. 32, p. 330; but the 

 wings are almo.st lost sight of in Vulsella. During later growth the hinge line becomes 

 twisted out of the straiglit line and commonly, for a brief extent at least, curves aroimd 

 the ligamental area as shown in fig. 36. 



The genera Malleus and Pinna might be considered in this section, but I have not 



Pig. 35. — Cassianella dfciissata, tip of loft valve, 

 showing, p, prodissoconch, and early dissoconch 

 arrowth. Original in Yale University Museum. X 30 

 diam. (Drawn by the author.) 



Fig. 3G. — Tip of valve of young 

 Vulsella rugnsn, showing, I, carti- 

 lage pit ; p, prodissoconch, succeeded 

 by the early nepionic growth. X 38 

 diam. (Drawn by the author.) 



