GEOLOGY AND PALEONTOLOGY OF THE CANAL ZONE. 399 



1863. Plesiastraea globosa Duncan, Geol. Soc. London Quart. Journ., vol. 20, p. 



38, pi. 4, fig. 5. 

 1917. Solenastrea bournoni Vaughan, U. S. Geol. Survey Prof. Paper 98-T, pp. 



372, 374, pi. 99, figs. 1, la, 16, 2, 3, 3a; pi. 100, figs. 1, 2, 2a, 3, 3a, 36. 



The following description is based on specimens from the Pliocene 

 Caloosohatchee marl of Florida: 



Corallum forming spheroidal or dome-shaped masses, sometimes 

 as much as a foot, or even more, in diameter; the outer surface 

 uniformly rounded or with gibbosities. 



The succeeding portion of the description is based upon a single 

 head-shaped specimen, 15.3 cm. tall; greater diameter 12.8 cm., 

 lesser, 11 cm. 



The calices have very slightly elevated margins, and thin corallite 

 walls. Diameter from 2 to 2.5 mm. Distance apart from 0.75 to 

 about 2 mm.; usually about 1 mm., or half the diameter of the 

 calices. The depth of the calicular fossae can not be determined with 

 certainty, as the specimen is worn; where it is best preserved they 

 are shallow. The corallite walls externally are costate, a costa cor- 

 responding to each septum; the costse, however, are short, those 

 from one corallite not extending to those of the next. Between the 

 corallites are thin-walled exothecal vesicles, which have a hori- 

 zontally stratified arrangement. The outermost exothecal platform 

 may show costal striations. 



The septa are thin, somewhat thicker at the wall; uniformly in 

 three complete cycles; primaries and secondaries equal and reaching 

 the columella; tertiaries only about half as long; thinner, inner 

 margins free. Rather wide, thin pali occur before the first and 

 second cycles. The septal faces are finely granulate, with the 

 courses of the trabeculae indicated; no perforations could be dis- 

 covered. Thin endothecal dissepiments present. Columella poorly 

 developed, rather small and lax. 



VARIATION OF SOLENASTREA BOURNONI. 



The United States National Museum possesses very large suites of 

 specimens of this species, permitting a rather satisfactory study of 

 its variation. The specimen already described shows within itself 

 the limits of variation in the size and distance from one another of 

 the calices. About 2 mm. is the average calicular diameter. The 

 exotheca may be very light and delicate, or rather compact, even 

 almost solid. The septa vary in thickness and the pali may be 

 strongly or weakly developed; where strongly developed they are 

 triangular in shape, the base of the triangle directed outward, and 

 the tertiaries may fuse to the basal corners or to the sides of the pali 

 before the secondaries. The thickened pali are correlated with the 

 denser exotheca, the various skeletal elements seem to thicken 

 together. 



