MONOGRAPH OF AMERICAN SHIPWORMS. 13 



BANKIA (NAUSITORA) EXCOLPA, new species. 

 GULF SHIPWORM. 



Plate 8, fig. 2 ; plate 31, fig. 4. 



Shell siibglobular, small. Umbones and the center of the middle 

 part tinged with rusty red. The anterior part with the usual sinus 

 and the reflected callus at the extreme anterior edge, the latter free 

 at its posterior termination on the outside. The anterior portion 

 is marked by a series of dental ridges which take a sudden bend 

 toward the callus near the anterior termination, and spread some- 

 what fanlike posteriorly. These ridges are about half as wide as 

 the spaces that separate them. In the type, which shows scarcely 

 any erosion at the umbone, 72 of these ridges are apparent. They 

 are triangular in cross section, sloping a little more abruptly on 

 the umbonal side than the ventral, and are finely denticulated at the 

 free margin. The anterior portion of the median part is rather 

 broad, and is marked with the dental ridges which are separated 

 by mere lines and join those of the anterior part in a little more 

 than a right angle. These dental ridges are provided with fine 

 denticles. The median portion is depressed into a shallow groove, 

 the anterior part of which is marked by the feeble nondenticulated 

 upward flexed continuations of the dental ridges which here become 

 decidedly reduced in strength. The posterior portion of the median 

 part is about as broad as the middle and anterior portion combined 

 and is marked by rather strongly impressed irregularly disposed 

 grooves. The posterior part forms a narrow auricle which joins the 

 posterior median portion in an even concave curve. It is marked 

 on the outside by very regularly spaced incremental lines. The 

 umbone is rather pronounced on the inside and the blade, springing 

 from the inner side of it, is moderately broad, sickle-shaped, curved, 

 and slightly flexed. The junction of the anterior and median por- 

 tion is denoted by a tumid area, while the posterior part passes over 

 a considerable portion of the median part with which it is fused, 

 being free only at the inner border. The central median portion 

 is marked by a groove. The extreme ventral point of the median 

 part is provided with the usual strong knob. The pallets have very 

 strong stalks and are covered on the outside by a strong shelly 

 deposit which in turn is covered with a periostracum. Wherever 

 this is broken it shows the lamellations beneath, in the blade. On the 

 inside, the blade beyond the smooth basal third shows a series, of 

 transverse laminae, each alternating with a dark cord of perios- 

 tracum. 



The type and a lot of other specimens, Cat. No. 98763, U.S.N.M., 

 come from a piece of Spanish cedar and were collected by A. W. 

 Crawford in the Gulf of California. 



03044—22 2 



