350 ROBERT TRACY JACKSON ON THE 



width of the shell. The base of the byssal notch bears a number of teeth like those of 

 Pecten irradians and Chlamys islandica, but they are wanting in the adult. 



Pecten testce. In the Jeffreys collection, now in the Smithsonian Institution, there are 

 two very young specimens of this species, catalogued as No. 62007. I sawthem in Wash- 

 ington, but did not borrow them as I did specimens of the above described species. From 

 a drawing made at the time (previous to my studies of other young Pectens) the upper left 

 valve exactly agrees m form and is of about the same size as Pecten irradians,V\. xxvin, 

 fig. 9. A prodissoconch like that of P. irradians and other species exists in my drawing 

 of the specimen. The early dissoconch shell has no ears and is comparable to ancient 

 forms of the Aviculidas as discussed in considering the same period of development of 

 Pecten irradians, with which it agrees in form as far as my drawing indicates. Minute, 

 punctate structure of the dissoconch was observed in this species similar to that noted 

 in Pecten dislocatus; but I cannot say whether prismatic structure exists although in all 

 probability it does. 



Specimens oi Pecten monotimeris, Con., recently received from California, have a sim- 

 ilar prodissoconch and nepionic stages to those described in P. irradians, as ascertained 

 by following the lines of growth. In the nepionic stage, the right valve has prismatic cel- 

 lular structure and the left valve punctate structure as in P. irradians, but as in that 

 species these featiu-es are early lost. 



XI. Genera allied to Pecten. 



Of the genera here considered I have had only limited opportunities to study the young; 

 for, as conchologists commonly pay little attention to the young of molluscs, it is a diffi- 

 cult matter to obtain early stages of genera not living in our own waters. 



Hinnites, PI. sxvi, figs. 3-4, is a genus closely related to Pecten, but as it has bee7i 

 described in section vii, I will give only brief sup piemen taiy notes at this point. It 

 is shown, p. 324, that Hinnites cortesi is pecteniform and regular when young; later it 

 solders one valve to a foreign body and with this change in habit assumes an irregularity 

 of growth rendering its valves, especially the attached one, comparable to Ostrea in form. 

 The form of the valves is ascribed to the condition of cemented fixation. The relation 

 of Hinnites to Pecten I believe is directly comparable to the relation of Ostrea to Perna; 

 the difference being that Hinnites in the nepionic and nealogic stages is free or byssated 

 and then retains the ancestral Pecten features. It does not become attached and assume 

 the ostx'ean irregular growth (wiping out early characters) until a comparatively late 

 stage of development, PI. xxvi, fig. 3. On the other hand, Ostrea is attached at the 

 close of the pi'odissoconch period and at once assumes the ostrean form, by this means 

 eradicating features which might otherwise render the early dissoconch growth compara- 

 ble to the ancestral Perna or Perna-like form. The age at which fixation takes jilace 

 varies somewhat in individuals, as may be seen in series of specimens of Hinnites gi- 

 ganteus. That species is instructive as showing how, in the development of the individ- 

 ual, the imbrications of the lines of growth in the nealogic pecteniform stage gradually 

 become more and more produced, until in the adult they form spinous productions of the 



