STRATIGRAPHIC PALEONTOLOGY. 101 



fauna and in places constituted reefs. The type locality of the formation is 

 at stations 8532 and 8533, opposite Mao Adentro, on Rio Mao, where 18 

 species of corals were collected. Station 8531 is on the same river farther 

 downstream. The stratigraphic relations of these exposures to the Gurabo 

 formation are entirely clear. The total number of species of corals col- 

 lected at the two localities is 26. The exposure at station 8663, on Arroyo 

 las Lavas, is correlated with the Mao Adentro limestone because of faunal 

 similarity, as about 14 out of 18 species (about 80 per cent) of the corals 

 collected at the type locality of the formation occur also at station 8663. 

 However, the total number of species obtained at station 8663 is about 34. 

 A comparison of the list of species of the Mao Adentro limestone with that 

 of the Gurabo formation shows that the faunas of the two formations have 

 many species in common; in fact there is very little difference between the 

 fauna of the upper part of the Gurabo and that of the Mao Adentro. The 

 strata exposed at stations 8556 and 8735, bluffs U and W on Rio Gurabo, 

 might, according to the fossils, be referred with almost equal propriety to 

 either of the formations. 



The molluscan fauna of most coral reefs is meager, and it is therefore not 

 surprising that only 19 species were collected from the Mao Adentro lime- 

 stone. Most of these were collected at Arroyo las Lavas (station 8663). 

 All the species determined except one, which is confined to this horizon, 

 occur also in the Gurabo formation. Although most of these species have 

 been found in the upper part of the Gurabo formation, Conocerithium 

 gurabense (Maury), the most abundant species at station 8663, occurs in 

 the lower part of the Gurabo formation at stations 8715 and 8519. 



As the Mao Adentro limestone is faunally so similar to the Gurabo forma- 

 tion it should probably be considered middle Miocene, slightly younger than 

 the Gurabo. 



Mao Clay. 



The fauna of the Mao clay is very meager. It includes one species of 

 coral and nine species of Mollusca. Only one of the determinable species 

 of Mollusca is confined to this horizon. All the other species occur in the 

 Gurabo formation, and one of them, Cancellaria (Bivetia) epistomifera 

 Guppy, is also found in the Cercado formation. There is no evident faunal 

 basis for recognizing the Mao clay as a separate formation. The reason for 

 assigning a distinctive name to it is given on page 75. 



Monte Cristi District. 



As it is not practicable to apply with certainty the formation names used 

 east of Rio Cana to the deposits examined by Mr. Ross in the vicinity of 

 Monte Cristi the collections made near that town are listed separately 

 from those made from the Miocene formations of the north side of the Repub- 

 lic. (See pp. 153-154.) The exposure at station 8780 represents a horizon 



