226 MESOZOIC AND CENOZOIC ECHINODEKMATA OF THE UNITED STATES. 



surface rising gradually from the margin to the moderately elevated, anteriorly excentric apex; 

 under surface flat. Ambulacral petals large, the posterior pair longer than the others. Apical 

 system excentric anteriorly. Peristome small, subcircular, excentric anteriorly; ambulacral 

 furrows simple and straight near the peristome, then forking, each pair of branches diverging 

 and again converging near the margin. Periproct very small, elliptical, between the inner end 

 of the odd lunule and the peristome. 



Dimensions. — Figured specimen, one of the largest found: Length 152 millimeters; width 

 159 millimeters; height 19 millimeters. Smaller specimen: Length 106 millimeters; width 111 

 millimeters; height 12 millimeters. 



Description. — The specimens herein described are believed to be representatives of the 

 recent species Mellita pentapora (Gmelin) which has been known under the names of M. testio- 

 dinata Klein and M. quinquefora Lamarck. Evidently, as Holmes recognized, the conditions 

 were especially favorable to development along the South Carolina coast during Pleistocene 

 time, a fact which is shown by the large size of the molluscan forms as well as by the Echinidee. 



As this well-known recent species is fully described in other works, it is necessary to give 

 here only the special points of interest in regard to the specimens occurring as fossils. Conrad's 

 species Mellita texana, of which what is probably the type is in the collection of the Academy 

 of Natural Sciences of Philadelphia, presents no marked and constant points of difference 

 from the recent species and therefore can not be recognized. Holmes's variety ampla is not 

 entitled to separate recognition and description, as, aside from the unusual size of the South 

 Carolina specimens, there are no decided and constant differences. Some specimens of the 

 so-called variety ampla present differences in regard to the lunules which make them appear 

 varietal in value, but a careful study of the large number of specimens in the collections of the 

 United States National Museum and the Johns Hopkins University has demonstrated that 

 the form of the lunules is so variable as to be unsafe as a feature for separation. This 

 species is very abundant and occurs in a beautiful state of preservation at a few localities in- 

 South Carolina, but is rare elsewhere. Most of the important features of the test have been 

 set forth in the determinative characters, but a few additional details will be given here. The 

 size of the fossil forms from South Carolina is very great, ranging from 4 to 7 inches in diame- 

 ter and from £ to J inch in height, while those from Texas are rather small, being between 2 and 

 3 inches in diameter, which is about the same as that of the average recent forms. The test is 

 usually slightly broader than long. The margin is truncated and especially thin posteriorly. 

 The lunules are long narrow slits of about the same -width from end to end, bluntly rounded 

 at the ends, the length and width quite variable even on the same specimen, the odd posterior 

 lunule usually wider and of equal length or slightly longer or shorter than the rest; the average 

 length in the South Carolina forms is about 1 inch and the width one-eighth of an inch. The 

 petaloid region is large; the petals are long and wide, subelliptical to subspatulate in form, 

 extending about halfway to the margin, well open at their ends, with very broad poriferous 

 zones which about equal in width the interporiferous areas. The apical system is large; madre- 

 porite large and stellate; four genital pores at the tips of the points of the star, the two pos- 

 terior farthest apart; five small pores at inner ends of reentrant angles of the star. The main 

 branches of the ambulacral furrows are more or less flexuous and each branch gives off one 

 important lateral branch as it bends to approach the margin. The undersurface, though flat, 

 in general, has shallow grooves along the middle of each ambulacrum. 



Related forms. — The only American fossil form which is closely related to the present 

 species is Mellita caroliniana which, however, , is readily distinguished by its having a sixth 

 lunule in the middle of the anterior ambulacrum, by having its ambulacral lunules nearer the 

 margin and its posterior paired lunules longer than the anterior pair; and having the relative 

 height of its test less than in M. pentapora. 



Localities. — Simmons Bluff, Youngs Island, Wadmalaw Sound, S. C; Brunswick Canal,, 

 Glynn County, Ga.; Texas ( ?). 



Geologic horizon. — Pleistocene. , 



