Paleontology Zorritos Formation 15 



as well as that of the other species, seems to demand a strictly 

 littoral zone as the habitat of most of the fauna. Most of the 

 Upper Zorritos Turritellas. for example, occur at Quebrada 

 del Toro, in a sandy matrix which does not bespeak deep 

 water bottom conditions. 



It is possible, however, that those horizons which are 

 notable for the larger, more diversified Turritellas, chiefly of 

 the Upper Zorritos formation, may represent at least tem- 

 porary occurrence of deep-water conditions. If that is the 

 case, the coast must have had a declivity sharp enough to 

 permit the carrying of somewhat coarse materials out into 

 deep water, and the run-off must have been at least locally 

 strong enough to wash sands and small pebbles some dis- 

 tance beyond the strand. It is entirely possible that periods 

 of flood on a steeply sloping coast may have carried the 

 coarse material found with many of the Upper Zorritos 

 fossils into comparatively deep water. 



The presence of a Trinmplm in the collection is of interest 

 in that the genus, as now recognized, contains but one other 

 species, and that living, restricted to the West Coast. The 

 Zorritos species is apparently the first of its direct ancestors 

 to be known. It is highly probable that a comparative study 

 of the genus Agasoma Gabb will show a relationship to 

 Trill mollis clo.ser than that now expressed, with the possi- 

 bility that Agasoma may have to go into synonymy. In the 

 latter event the genus will have a broader range, with more 

 fossil species. 



The remaining gastropods are almost uniformly of groups 

 naturally to be expected in a tropical American Miocene 

 fauna, and the chief interest they offer lies in their geologic 

 affinities. One species of Solcnostcira is interesting in that 

 it is of a type not widely known from the Miocene, but 

 develof)ed to some degree in the recent fauna of the West 

 Coast. A single species from Gatun is the only other known 

 in the Miocene. The ancestors of the recent forms are thus 

 found in the Panamanian — Peruvian region. Other fossil 



