42 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 30 Ser. 



Among the gasteropods of the Horsetown which may be 

 mentioned are species of Nerinea, RingincUa, Actaon, 

 Anchu?'a, and Helicaulax. Bivalves are represented by 

 such forms as Avicula, Plciiromya and Lima, two species 

 each, along with others, as Plicatula varia and Thelis clon- 

 gata. The following also are typical Horsetown spe- 

 cies: Crioceras latum, C . percostatiim, Diftychoceras Icbvc, 

 Schlocnhachia injlata, Liociiim ^unctatiim, Potamidcs dia- 

 denia (?), Oxytoina imicronata, Ai'chomya undtdata and 

 Mithea grandicosta. More than eighty species in all are at 

 present known, though it is quite probable that this is not a 

 large part of what will be known when the beds are more 

 carefully searched. 



The Horsetown fauna in its most typical development is 

 of a tropical character, as has already been noticed by sev- 

 eral writers. Many of its congeners are numerous in the 

 fauna of Southern India. Both have evidently come essen- 

 tially from the same source. The southern aspect of the 

 Horsetown is seen in the numerous species of Lytoceras, 

 Phylloceras, and many of the crioceran and nautilian 

 forms. In this respect it contrasts strongly with the 

 northern aspect of the fauna preceding it, in the upper 

 portion of the Knoxville. 



Comparatively few of the gasteropods and bivalve species 

 occurring in this list continue above the Horsetown, though 

 some have allies even in the Upper Chico. Probably when 

 the Horsetown fauna becomes more completely known the 

 transitional forms will appear even less significant, since the 

 cephalopods form its ruling class. Perhaps, also, it will be 

 possible to separate it into subdivisions, better characterized 

 than those of the Chico. Diller and Stanton (1894, P- 445) 

 mention as belonging to its upper portion only, Lytoceras 

 sacya, Desmoceras beiidanti, Schlcenhachia injlata, Acanthoc- 

 eras mamillarc and a few other forms. Likewise there are 

 a few that belong especially to the lower portion of the 

 Horsetown, among which are Belemnites impresses, Crioc- 

 eras percostatiim, Olcostephaniis traski, and perhaps Heli- 

 caulax hicarinata and Potamidcs diadema. On the other 



