510 PROCEEDINGS OF THE ACADEMY OF [Dec, 



THE PR0T0C0NCH OF ACM.EA. 

 BY WILL F. THOMPSON. 



The protoconchs or embryonic shells of the Mollusca have been 

 used by various writers as indicating the status of the various 

 groups and their line of descent. The Acmaeidse have many primi- 

 t ive characters, and it would be expected that the embryonic stages 

 of the shell would show a like condition. It has been stated that 

 these forms possess a coiled nautiloid protoconch by Grabau. '03; 

 Pelseneer, '06; Fisher, '04; Verrill, '96, and others. In a recent 

 article by Morse, '10, this is declared erroneous, and figures and 

 descriptions are given of a stage of Acmcea testudinalis in which there 

 is shown "simply a csecal-like shell with slight dorsal flexure" 

 and "a slight elongated area rounded anteriorly" at either side 

 where the embryonic shell joins the permanent. 



The specimens of Acmcea cited by Dr. W. K. Fisher were given to me 

 by Dr. Harold Heath, with the request that I investigate them. I 

 thank him for his kindness, as well as Dr. J. P. Smith. 



The material was imbedded in very hard paraffin, either entire 

 or in part decalcified, and sectioned. This method gave but poor 

 results, and better were obtained by observing the specimens in 

 reflected light under high powers of the microscope. By carefully 

 working over the debris picked up with the young limpets, decol- 

 lated shells were found without the embryo within them and hence 

 iii excellent condition for observation. All the material came from 

 the tide pools of Monterey Bay, but the species could of course not 

 be certainly identified. 



The general appearance of the shell may be seen in fig. 1. A 

 slight asymmetry is visible, the posterior apex of the protoconch 

 being to the left of the adult axis, although symmetrical to that of 

 i In larva itself. In view of the presence in nearly related mollusks 

 of naticoid and planorbid coils, this probably has but very little 

 significance. The " lateral folds " of Morse, TO, are very evident 

 and easily seen (fig. 2), leading, if seen from one side, to the appear- 

 ance of a true spiral coil, but present on both sides. The right-hand 

 one is in the greater number of cases slightly longer than the left, 

 in harmony with the slight asymmetry. As may be seen in figs. 



