ORTMANN: A MONOGRAPH OF THE NAJADES OF PENNSYLVANIA 283 
almost exclusively by Lea and Simpson. The latter in his great ‘“Synopsis’’ (1900) 
gives generalized accounts of the soft parts, chiefly in the diagnoses of the higher 
groups, genera, and their subdivisions; but we often remain in doubt whether a 
particular species has been examined by Simpson or not. Simpson furnished de- 
scriptions of the soft parts of species for another publication (See Baker, 1898), 
but in this case it is to be observed that he sometimes described the soft parts of 
a species which he did not examine, or rather that he inferred some of the char- 
acters from other species, which he believed to be allied. Quadrula coccinea is an 
instance. The description of the marsupium is here positively incorrect and was 
introduced apparently in the belief that this species, being supposedly a Quadrula, 
should have the marsupium of this genus. 
Nevertheless, Simpson’s observations published in the “Synopsis,” 
are of the 
highest value, for he correctly recognized the most essential part of the anatomy, 
namely the marsupium. In all Unionide (as in the Najades in general), the eggs 
are carried in the gills, where they develop into larve, known as glochidia. This 
is a well known fact; and it was also long ago known (chiefly through Lea) that 
the marsupium is formed in the different species by different gills, or parts of gills. 
But this character had never been employed in making a systematic arrangement, 
and the credit for recognizing the importance of the marsupium, and for using 
it successfully in the division of the Unionide into genera and in the grouping of 
the latter, distinctly belongs to Simpson. 
But Simpson studied only the general features of the marsupium, and thus 
his groups are not in all cases without objection. Further in a number of species 
he did not succeed in ascertaining the shape of the marsupium, and thus was able 
to place them only tentatively. As has been said above, several well supported 
corrections have been introduced by Sterki (1903), who paid attention chiefly to 
the shape of the glochidium, together with that of the marsupium. 
The present writer made it his chief purpose to ascertain the shape of the 
marsupium in all species which came under his observation, and to this end to 
obtain gravid females. In this undertaking he was generally successful, and, with 
very few exceptions, gravid females of all Pennsylvanian species were secured. 
But when he took up the closer anatomical study, he made a very important 
discovery, namely that in all our Unionide the anatomical structure of the gills, 
which serve as marsupia, is permanently differentiated. It was known and empha- 
sized by Sterki that this is externally the case in a certain group of genera (Lamp- 
silis-type), while it was maintained that in the rest no such differentiation was 
visible in the female, when not gravid. But this latter assumption is incorrect. 
