b THE NAUTILUS. 



the present communication is to point out the essential features of 

 the anatomy, as far as they are of systematic value. This I deem 

 an urgent necessity, since to all appearance the anatomy and the 

 systematic relationship of the European forms is not very well under- 

 stood among the European writers. 



Family MARGARITANID^; Ortmann. 



The chief differences of this family are found in the incomplete 

 gill-diaphragm ; in the entire lack of a tendency to approach or to 

 unite the mantle margins to form siphons ; in the structure of the 

 gills, which lack regular septa running parallel to the gill-filaments, 

 and consequently, in the lack of the well-defined water-tubes (ovisacs 

 in the female); in the marsupium being formed by all four gills ; and 

 in the small size and globular shape of the glochidium, without true 

 hooks. 



The only known genus is Margaritana Schumacher. 



Margaritana margaritifera (Linnaeus). I have the soft parts of 

 a specimen from Thuringia (Elster River drainage). 



The soft parts agree in all essential points with North American 

 specimens of this species collected by myself in Schuylkill Co., Pa. 

 1 have considered the identity of the species as established. 



Margaritana sinuata (Lamarck). This species is not before me. 

 It is the Margaritana crassa of Simpson (P. U. S. Mus. 22, '00, p. 

 667). The mistake of Simpson in using the specific name cras- 

 sus Retzius for this species has been amply demonstrated by Kobelt 

 (Zur Kenntnis unserer Unionen, in : Festschr. 100-jaehr. Best. 

 Wetterauisch. Ges. Naturk. Hanau. 1908, p. 99, and : Simpson und 

 die europaeischen Najaden, in : Beitr. Kenntn. mitteleurop. Naj., 

 Beil. Nachr. Bl. deutsch. malakozool. Ges. no. 3, 1909, p. 43). Re- 

 cently, Haas (Nachr. Bl. etc., 4. 1910, p. 181) has created the new 

 genus Pseudunio for this species, and gives a description of the soft 

 parts. Unfortunately he fails to say anything about the gill-struc- 

 ture. But as far as this description goes, it is absolutely identical 

 with that of the soft parts of Margaritana margaritifera, and the few 

 differences of these two species pointed out by Haas I cannot regard 

 but as specific differences. Haas apparently overestimates the differ- 

 ences, while he underestimates the resemblances, aside from neglect- 

 ing entirely one of the most important characters (gill structure). 



Much stress is laid upon the presence of lateral hinge teeth in 



