AET. 18 NEW MOLLUSKS FROM TEXAS — STEPHENSON 3 



Beaks relatively small, projecting slightly above the hinge, gen- 

 erally curved over to the left from the plane of contact of the two 

 valves. 



Left valve flattish to slightly convex, smooth, with the exception of 

 fine growth lines, some stronger growth lamellae, and irregularities 

 due to crowding ; scar of attachment relatively small, the oyster show- 

 ing a decided preference for attachment to the shells of an elongated 

 mollusk, probably GervilUopsis ; margins overlapping the margins of 

 the right valve. 



Eight valve smaller, flatter, and smoother than the left valve, and 

 wing projections less pronounced. 



Hinge triangular, the base a little longer than the sides ; ligamental 

 groove more deeply impressed on the left than on the right valve; 

 adductor scars of moderate size, situated high, and toward the poste- 

 rior margin of the shell ; a small pedal muscle scar is just below the 

 lower end of the ligamental groove; on some specimens irregular 

 striations appear on the inner surface where the upper ends of the 

 posterior and anterior margins of the right valve fit against the left 

 valve just below the hinge. 



Remarks. — This species is unique among the many oysters in the 

 Cretaceous deposits of the Atlantic and Gulf Coastal Plain. Though 

 simple, plain, and of moderate size, it presents characters which un- 

 mistakably distinguish it from all other species. It has been found 

 only in one zone a foot or two in thickness, along a linear distance 

 of nearly 50 miles. It appears to have no ancestors in the older 

 Cretaceous deposits of the Coastal Plain and, so far as known, it left 

 no descendants. It appeared suddenly in an environment that was 

 evidently favorable, for the individual shells are numerous. It re- 

 mained only for a moment (in a geologic sense), and for some un- 

 known reason disappeared as suddenly as it had come. The most 

 probable fatherland for the immigrant stranger is perhaps the trop- 

 ical seas of the Caribbean region, the Cretaceous history of which is 

 imperfectly known. If this be true, the cause of the appearance and 

 disappearance of the species might be surmised to have been a tem- 

 porary warming up of the waters of the Gulf region, followed by a 

 cooling off of the waters to a degree unfavorable to the survival of 

 the young oysters. 



Types.— Roloiy^Q: Cat. No. 73657, U.S.N.M., from a ravine 234 

 miles west-northwest of Sparks, Bell County. (See below.) 



Paratypes : Cat. No. 73658, U.S.N.M., from Cottonwood Creek, 2.7 

 miles north-northwest of Hutto, Williamson County ; Cat. No. 73659, 

 U.S.N.M., from Little Walnut Creek, 214 miles southwest of Sprinkle, 

 Travis County. (See below.) 



Distribution. — Upper part of Austin chalk: On the downthrow 

 side of a small fault, in a ravine 2% miles west-northwest of Sparks, 



