THE NAUTILUS. 103 



the marsupium the Pleurobema-Unio type, but differs by a general 

 tendency to reduce the hinge teeth. Another group is formed by the 

 rest of the genera, in which the marsupium becomes very highly 

 specialized, more or less restricted to a part of the outer gills, and 

 where true ovisacs are developed. All these more advanced genera 

 originated probably at a time when seasonal changes of climate 

 existed already in our continent — in the tertiary — and the shorten- 

 ing of the warm period in summer possibly induced them to prolong 

 the breeding season, that is to say, to postpone the discharge of the 

 embryos to a more favorable time, namely, till the next spring. 

 This made necessary special adaptations for the carrying of the em- 

 bryos through the winter, and probably the ovisacs of the most 

 highly developed genera belong to these special adaptations. In 

 certain genera, ovisacs are not at all developed, and in Strophitus an 

 independent form (placentae). This lengthening of the breeding 

 season finally led to the merging of the end of the one of them into 

 the beginning of the next (known only in one of the most highly 

 specialized genera, Lampsilis), while in less specialized genera, in 

 Alasmidonta, Symphynota, Anodonta, also in Ptychohranchus and 

 some species of Lampsilis, an " interim " in midsummer still exists. 



I think this is a reasonable interpretation of the different types of 

 breeding season and their development, yet it is proposed here as a 

 mere theory, which should be substantiated by further investigations 

 on the marsupium and the breeding seasons of our Unionidce. 



MOLLUSES FBOM ABOUND ALBHaUEBQUE, NEW MEXICO. 



BT H. A. PILSBRY AND J. H. FERKISS. 



A considerable amount of work has been done on New Mexican 

 mollusks, and the numerous local lists published would make a good 

 showing for the Territory if compiled into one catalogue. The more 

 recent lists were based upon material collected by Professor T. D. A. 

 Cockerell and his pupils, and by Messrs. Joshua L. and Albert Baily, 

 Ferriss and Pilsbry. Tlie records are to be found in Nautilus, ix, 

 p. 116; X, p. 42; xi, p. 69; xii, pp. 76, 131; xiii, pp. 13, 36, 49, 79; 

 xiv, pp. 9, 47, 72, 82, 85; xvi, pp. 57, 69, Mollusca of the South- 

 western States, I, II, etc. 



