GEOLOGY AND PALEONTOLOGY OF THE CANAL ZONE. 441 



conspicuousl,y open. Synapticulac are present, but they are rather 

 scarce, and are delicate. The delicate wall and synapticulae and the 

 relative openness of the interseptal loculi constitute striking differ- 

 ences from the appearance presented by S. radians. 



Locality. —S&nto Domingo, collected by W. M. Ga})b. 



Type. — Museum of Comparative Zoology. 



6. SIDERASTREA PLIOCENICA, new species. 



Plato 118, figs. 2, 2a, 26, 3. 



TAvelve specimens, all of them excellent, serve as the basis of the 

 following specific diagnosis. One is designated as the type in the 

 collection. 



The corallum usually forms a rather small rounded head, but a 

 few are elongate, and one is fiattish, sublamellate. The heads attain 

 a diameter of between 45 and 50 mm. About a third of the speci- 

 mens show signs of having been attached or have not calices uni- 

 formly distributed over the whole outer surface of the corallum. 



The corallites are rather large, and are rather uniforml}^ hex- 

 agonal or pentagonal; usual diameter is 4.5 to 5 mm.; intercoralhte 

 wall distinct and zigzag in plan. The calices are shallow or super- 

 ficial. 



Septa thick, usually in almost four complete cycles, the fourth 

 cycle is as a rule absent in one or two systems. Septal margins 

 dentate, each dentation rounded, corresponding to the upper termi- 

 nation of a septal trabecula, the number of dentations on a septum 

 of the first cycle varies from 8 or 9 to 13. The length of such a 

 septum is almost 2.5 mm. Septal grouping is as usual in the genus^ 

 the members of the first cycle are continued directly to the colu- 

 mellar space and do not form parts of septal groups; the members 

 of the second cycle, also, are continued directly to the columellar 

 space, but each member of this cycle is the middle of a septal group, 

 the members of the third cycle bend toward it, and the members 

 of the fourth bend toward the included member of the third. Along 

 the course of each trabecula is a regular row of granulations, which 

 are compressed in a plane transverse to the longitudinal course of 

 the trabecula. Septal perforations are frequent near the inner 

 margins of the septa, usually occurring in the intertrabecular spaces, 

 but in places a large perforation interrupts a trabecular course. 

 The perforations become rarer as the wall is approached. Com- 

 pletely imperforate septa are very rare or do not exist at all. 



Both synapticulate and dissepimental endotheca is present. In 

 places as many as four or five vertical rows of synapticulae can be 

 distinguished. Ver}^ tliin dissepiments are abundant. The wall is 

 formed by synapticulae that are so elongated in a vertical row that 



