WILCOX GROUP. 



31 



of waters not exfcodiiiji; 1.') to 25 fathoms in 

 <loj)tli. The characteristic features of the 

 niaki'-iij) of the fauna are probably due, quite 

 as much to the reconnaissance nature of the 

 work done upon it us to the actual facias. 

 Early collections inchuU^ as a rule only those 

 forms conspicuous cither for tlicir large size 

 or their great abundance, so that the notably 

 meager representation in the Nanafalia check 

 lists of all the genera of lesser dimensions 

 suggests that they formed an inconspicuous 

 factor in the original fauna. Fully 70 per cent 

 of the species recorded are univalves of medium 

 or large size — fulguroids, fusoids, l)uccinoids, 

 Pseudohva, Turritella, and the like — genera 

 mostly characteristic of the laminarian and the 

 sublittoral zones. The jielecypods lunnber less 

 than a dozen species, l)ut one of them, the 

 Ostrea thirsx of Gabb, is so C(>ns])icuously 

 prolific that the horizon is commonly known as 

 the Ostrea thirssc bed. The abundance of this 

 oyster not only conclusively estabhshes the in- 

 shore character of the deposits but also serves 

 as a fairly accurate Nanafalia horizon marker. 

 The affinities of the fauna as a whole are vague 

 and generalized. Of the 40 or 41 species and 

 varieties only 1 species and 1 variety are re- 

 stricted to the Nanaf aha formation. Of the 39 

 remaining forms, 9, or approximately 2.5 per 

 cent, range from the Midway to the upper 

 Wilcox or higher and have no significance in 

 close correlation; of the 30 Nanafalia species 

 occurring at not more than two liorizons only 

 4, or 13 per cent, are restricted to the Midway 

 and Nanaf aha; the other 26, or approximately 

 87 per cent, run from the Nanafalia up to a 

 higher horizon, of which 5, rouglily 19 per cent, 

 are not known except in the Nanafalia and 

 Tuscahoma formations. The fauna is, there- 

 fore, obviously Wilcox in its alHnities and 

 marks the initiation of many of the mi^st 

 characteristic Wilcox univtdves ])ut cxliibits 

 no peculiarly close relationship with any one 

 of the later horizons. 



The Tuscahoma is generally rather l)arren, 

 but at Bells Landing and Greggs Landing, on 

 Alabama River, and at Tuscahoma, on Tom- 

 bigbce River, extensive collections have been 

 made and worked up by Aldrich, Harris, and 

 others. The check lists consulted record 

 168 species. Of these species 121, or approxi- 

 mately two-thirds, are univalves. The fauna 

 is well diversified, includes botli herbivorous 



and carnivorous gastropods, and indicates 

 rather warm, shallow waters, not exceeding 40 

 fathoms, abounding in plant and aiiiniid life. 

 The unusiially lai'ge size attained by the indi- 

 viduals collected at Bells Landing' suggests a 

 pecidiarly favorable environment, in which 

 the inhabitants existed under optimum con- 

 ditions. The best represented geiu'ra are 

 among tiie larger unividves, nota])ly the 

 fusoids, fulguroids, tritons, and Cassidae. 

 Among the bivalves tlie taxodonts are rela- 

 tively rather numerous. Corbula also occurs 

 in considerable numbers along vdth the id)i((ui- 

 tous Venericardia. Tlu^ Tuscahoma, unlike 

 the Nanaf.dia, is clearly differentiated from 

 the formations abovc^ and below. Of the 165 

 spe(ues recorded 7!), or nearly 50 per cent, are 

 restricted to the Tuscahoma. Of the remain- 

 ing So forms 35 range both above and below 

 the horizon and may be disregarded by the 

 stratigrapher, leaving 49 species to be consid- 

 ered. The Tuscahoma mafks the lower limit of 

 range of 33 of these and th(> upper limit of range 

 of 16. These figures may appear more signifi- 

 cant than they really are, for the Bashi fauna, 

 whicli succeeds the Tusciahom^i, has been stud- 

 ied in much greater detail than the Nanafalia 

 below it. However, the critical work that has 

 been done on the Bashi makes all the more nota- 

 ble the fact that almost half of the species 

 recorded from the Tuscahoma are peculiar to it. 

 The Bashi formation ("Woods Bluff") pre- 

 sents the largest ami most diversified fauna ^ 

 known from the Wilco.x, notwithstanding the 

 entire lack of evidence of any a])])reciable 

 increase in the depth of the water. On the 

 contrary, the larger univalves, the fusoids, 

 tritons, and Cassida\ are relatively less numer- 

 ous tlian in tlie lower Wilcox. Many of the 

 smaller genera, however, abundantly repre- 

 sented in the littoral and laminarian zones of 

 our recent seas ocH'.ur in very consid('rabl(\ num- 

 bers. For example, 20 species of pleuroto- 

 niids, 3 of Canc.ellaria, 2 of Nassa, 2 of Ceri- 

 tliium, and 3 of Corbula. Th(> decrease in the 

 number of the genera pecidiarly characteristic 

 of the sublittoral zone is, however, merely rela- 

 tive, for 8 species of Fusus, 4 of Acteon, and 4 

 of Volutida) have been recorded. Tii(> change 

 in the chai'acter of the fauna is probably due 

 not to any marked change in the depth of the 



'Aldrich, T. 11., Alabama Oeol. Survey Bull. 1, pp. 54-5.'j. 1886. 

 2 This fauna has recenlly been discovered in east-central Mississippi 

 by E. N. Rose. 



