128 THE NAUTILUS. 



the anal opening is defined above by a growing together of the man- 

 tle margins, but here a supra-anal is never formed, the fusion of the 

 mantle edges being complete up to the upper posterior end of the two 

 mantle halves. In addition, we find in certain forms of this group a 

 further step in advance, which consists of the anterior demarcation 

 of the branchial opening by a growing together of the mantle edges. 



(c) The formation of the marsiipium ofTers the greatest variety. 

 It is hard to say which is to be regarded as the primitive condition, 

 but probably originally all four gills served as receptacula for the 

 ova, and the space between, the two laminas of each gill was not 

 diflferentiated and divided. 



Out of this original condition more advanced structures have de- 

 veloped, which generally exhibit the tendency to localize the mar- 

 supium in certnin gills or parts of gills, and to divide the interlaminar 

 space into compartments (ovi-sacs and water tubes). In the differ- 

 ent groups these purposes have been accomplished in different ways 

 and to diflferent degrees. In Margaritana the gills do not possess 

 partitions, and, consequently, are primitive in this respect, and ap- 

 parently no water-tubes and ovisacs are found. Unfortunately, the 

 gravid female of this form is unknown to the writer. 



In the more primitive Unionidce (subfamily Unionince), either all 

 four gills still serve as marsupium, and have thus preserved the orig- 

 inal condition, or only the outer gills serve this purpose, and further,' 

 here the interlaminar space is divided by septa into rather regular 

 compartments, running vertically to the edge of the gills, and parallel 

 to the gill filaments. The same fundamental characters, restriction 

 of the marsupium to the outer gills and development of water tubes 

 and ovisacs, are found in the more highly-developed Unionidce 

 (subfamilies Anodontina and Lampsilmce), but here specializations 

 are met with, which are to be regarded primarily as adaptations to 

 a prolonged breeding season, and to a peculiar way of discharging 

 the glochidia. 



In the Afro-South American group of Najades (^Mutelidce ?), all 

 cases so far known show the marsupium restricted to the inner gills. 

 Two main types may be distinguished among them, according to the 

 inner differentiation of the marsupial gill. In one case, the mar- 

 supial part of the inner gill is rather restricted (to the middle por- 

 tion), and here very incomplete and intercommunicating water tubes 

 are formed by rows of interlamellar connections ; in the other case. 



