GLOCHIDIA OF FRESHWATER MUSSELS. 5 



While all the species figured are not of uniform development, no 

 change of form nor increase in size would occur, except as above 

 noted. For instance, -S'. costata (fig. 7, pi. i) is more developed 

 than L. suhrostrata (fig. 16, pi. ii), and this again is greatly advanced 

 over^. granifera (fig. 19), the anterior and posterior adductor muscles 

 having become completely separated in costata, less so in suhrostrata, 

 and just beginning to separate in granifera. The shape and relative 

 position of the adductor muscle before separation is a uniform 

 feature for each species, and its importance as an aid to identification 

 should not be overlooked. 



Sterld^some years ago (1903) pointed out the character of the 

 glochidium as an important factor in the classification of the Unionidie, 

 and this is clearly confirmed in the case of L. anodontoides and L. 

 faUaciosa, the adult shells being very often inseparable, in fact, their 

 identification as separate species even under the most favorable cir- 

 cumstances being difficult. When we come to examme the glochidia, 

 however, we find that there is not only a difference in form but also 

 in size, L. anodontoides (fig. 21, pi. ii) being smaller and sHghtly 

 shorter in proportion to its dei)tli than L. faUaciosa (fig. 22, pi. ii). 



Owing to the small size of the glochidium of L. gracilis, and, not- 

 withstanding its affinity with L. (Proptera) alata in the structure of 

 the soft parts of the adult animal, Ortmann^ (1911) created for it a 

 new genus — Paraptera. If size and general shape alone were the 

 controlling factors then the very minute glochidium of both Plagiola 

 donaciformis (fig. 29, pi. ii) and P. elegans (fig. 30, pi. ii) would place 

 them with gracilis were it not for the gaping margins of the glochidial 

 shell in gracilis,'^ in which respect it resembles P. securis. The position 

 of these two forms {donaciformis and elegans), in my opinion, re;nains 

 in doubt, and the acquisition of more material, with careful study, 

 will probably reveal much of interest in relation to these small 

 mussels. 



It is unfortunate that more is not known as to the period during 

 which the Unionida^ are gravid, or rather as to when they carry 

 well-developed glochidia. Unfortunately investigators are not in the 

 habit of giving us uniform data in this respect, the term "gravid" 

 having too wide -a range of meaning and including too often mussels 

 which we may designate as bearing early embryos, late embryos, or glo- 

 chidia. In the case of the short-period breeders it does not matter 

 so much, for the period is so brief — a month, or two months at most — 

 that some fair idea may be formed of the date on which to expect 



a Sterkl, v.: Notes on the Unionldae and their classification. American Naturalist, vol. 37, p. 103. 



b Ortmann, A. E.: A monograph of the Najades of Pennsylvania. Memoirs of the Carnegie Museum, 

 vol. IV, p. 334. 



c Coker, R. E., and Surber, T.: A note on the metamorphosis of the mussel Lampsilis Itevissiinus. 

 Biological Bulletin, vol. xx, p. 180, and pi. 1, fig. 2a. 



