OBOLID^. 553 



ridge is large, only a trace of an outer groove remaining; in some young shells the groove is 

 broad and shallow; in all sliells the large size of the main vessels is shown by the broad, strong 

 grooves or ridges left on the shell. It frequently happens tliat the lines of sharply defined pits 

 on the lines of growth deeplj^ indent the grooves and rormded ridges left by the main vessels 

 and mark them off into sections. The interior and lateral vessels left narrow but strong grooves 

 or ridges on the shell, which, however, are usually obscured by the strong pitting of the surface. 

 The parietal scar surrounds the visceral cavity in each valve, crosses the course of the main 

 vascular vessels, and comes back around the spaces occupied by the muscle scars, terminating 

 at the edge of the area at the flexure line in the ventral valve; termination unknown on the 

 dorsal valve. 



Some of the muscle scars are finely shown in the dorsal valve and fairly well in the ventral. 

 The umbonal scar of the ventral valve is divided, the pedicle scar being situated between the 

 two parts. In the dorsal valve the umbonal scar is close to the area, and extends nearly as far 

 each side of the median hne as the length of the area. 



The scars of the central muscles in the ventral valve are crowded in with the middle and 

 outside laterals within the trapezoidal space (c, PI. XLIII, figs. Iq and Is). In the dorsal 

 valve they are located on a low ridge each side of a central, longitudinal median depression; 

 they are elongate oval in outline, their major axis being subparallel to the median line of the 

 shell; fine longitudinal lines cross the scars in the best preserved specimens; the ridge on wMch 

 the central scars occur varies in strength, but it appears to be present in all adult shells; it 

 narrows gradually posteriorly and rather rapidly to the inner side of the anterior lateral muscle 

 scars. The anterior laterals of the ventral valve are placed well back on the narrow space 

 between the edge of the area and the main vascular sinus; they are elongate and rather large; 

 in the dorsal valve they are elongate with the major axis inclining toward the median line (PI. 

 XLIII, figs. Ir and ly). The middle and outside laterals are situated in the trapezoidal area 

 (c) of the ventral valve, but neither is clearly separable fi-om the other or from the central 

 scars. In the dorsal valve the position of the middle and outside laterals is shown, but not 

 their form or size. The transmedian scars in the ventral valve are seen just back of the anterior 

 laterals, but they have not been observed in the dorsal valve owing to the imperfections of the 

 shell. 



Observations. — This is one of the most interesting species of LinguJepis I have knoAvn. In 

 its coarsely pitted inner surface it recalls Lingulella davisi (McCo}^) (PL XXXI) of England, and 

 Oholus willisi (Walcott) (PL XXIII) of the southern Appalachians. The elongate ventral 

 valve is hke that of Lingulella {Lingulefis) acuminata (Conrad), whUe the interior scars and 

 markings are those of Oholus. With L. (L.) gregwa (Matthew) and L. (L.) starri (Matthew) it is 

 the Atlantic coast representative of L. (L.) acuminata which is so abundant in the Mddle 

 Cambrian of the Upper Mississippi VaUey, and in the passage beds between the Cambrian and 

 the Ordovician in the region adjoining the Adirondack Mountains of New York. It differs 

 from L. (L.) acuminata in its surface characters. 



The shells embedded in the fine-grained shales are all flattened and more or less distorted by 

 compression, while those in the sandy layers interbedded in the shales retain their original 

 convexity, although fi-equently distorted and fractured by movement of the matrix. The 

 fracturing of the brittle, mineralized shell is beautifully illustrated by Plate XXXIII, figure 

 4a, which represents the sOiceous fillings of the fi'actures, the shell substance having been 

 dissolved with acid. 



Typical specimens of Lingulella (lAngulepis) exigua have narrow, elongate ventral valves and 

 broadly oval dorsal valves. I was at first strongly inclined to separate the broad, thick shells 

 as a distinct species, but on finding a series of connecting forms and observing that the surface and 

 interior markings were similar, I decided to unite them. The shells vary materially in their 

 external aspect in the different layers of siliceous rock, but when the shell substance is removed 

 by acid it is found that their interior and exterior characters are the same. The tj'pical forms 

 of L. (L.) exigua are represented in Plate XLIII, by figures 1, la-b, Id, Ig-j, and the variations 



