OF CONCHOLOGY. 161 



in size and coloration, but which alone cannot be regarded as of 

 specific value. The color of the Nicaraguan shell is yellow horn, 

 without green bands, shining, and marked with faint radiating 

 lines. The dimensions of the largest specimen are as follows : 

 Total length 3-35 inches; height -95 inch; thickness "5 inch; 

 length of anterior side '9 inch. 



M. Weddelli was collected on the muddy margins of the river 

 Tipitapa. 



Collecting Unionidce in tropical countries can only be success- 

 fully pursued during the dry season, as during the rainy season 

 the species are in comparatively deep water, and cannot be gath- 

 ered except by means of a dredge, the employment of which 

 under such circumstances I would recommend. An examination 

 of rivers and lakes at the two seasons will give widely different 

 results as regards the presence and abundance of these forms of 

 molluscan life. The Unionidce inhabiting large sheets of water 

 are presumed to be littoral ; certainly in Lake Nicaragua the 

 upper limit of their distribution is that of low-water mark, but 

 their extension in depth is not known. All the species I have 

 recorded are littoral, but whether they are confined to this zone, 

 or that other species replace them in deep water, are unsolved 

 problems. The thin-shelled species are largely preyed upon by 

 herons and other birds, and often much to the annoyance of the 

 collector. During my travels in Venezuela once only did I meet 

 with Unionidce^ — on the margins of a lake-like expansion of the 

 river Carichapo ; but not a single perfect shell did I secure, 

 though innumerable fragments strewed the shore, they having 

 broken all the valves left by the lowering of the level of the 

 water. 



The following molluscs recorded from Nicaragua did not occur 

 to me : Helix Parkeri, Helicina merdigera, Cyrena sordida, Unio 

 aratiis, Unio cyrenoides, Unio Newcombianus, Anodonta Nica- 

 ragua, 



III. Relation to other Faunas. 



The molluscan fauna of Nicaragua presents no marked facies, 

 and is characterized by the absence of, rather than the presence 

 of peculiar genera. The geographical position of Nicaragua leads 

 us to infer that the species of shells would be in common with 

 those of the Mexican province on the one hand, and with those 

 of the Columbian province on the other, and such is the case. 

 Thus, whilst Bulimus Bercndti, B. unicolor, B. maculatus, B. 

 mimosarum, Planorbis Fieldii, Cyclotus translucidus, Arnnicola 



