470 CAMBKIAN BEACHIOPODA. 



In the anterior portion of the visceral area of the dorsal valve of some species the "heart- 

 shaped pit" of Oholus is distinctly marked (PI. XVII, fig. If). It is rarely so well shown, 

 although its anterior outline is preserved in nearly all species whose interior characters are weU 

 marked. A median ridge may or may not be present in the interior of the dorsal valve. 



The preservation of all vascular markings depends largely on the thickness of the shell, 

 being rarel}^ seen in thin shells and often found in thick ones. 



There are six pairs of muscle scars that have been observed, in addition to the pedicle scar 

 (m) which is situated on the ventral valve between the two scars (gg) left by the divided umbonal 

 muscle close to the area; on the dorsal valve the single umbonal scar (g) is just in advance of 

 the base of the area. The central scars (h) are usually situated about the center of the dorsal 

 valve a short distance each side of a median septum; in the ventral valve they are inclosed 

 in a trapezoidal area (c), with the scars of the outside and middle lateral muscles, on each 

 anterolateral side of the heart-shaped area (x). The transmedian scars are shown as one 

 scar on each side a little in advance of and on a line with the flexure lines of the area; the 

 variation in their relative position in the two valves and in various species may best be under- 

 stood by studying the illustrations and descriptions of species. The scars (k) of the middle 

 lateral muscles are inclosed within the area (x) with the central and outside laterals; in the 

 dorsal valve they blend with those of the outside laterals (1) in advance and a little outside of 

 the transmedian scars (i). As previously stated, the scars of the outside lateral muscles in the 

 ventral valve are combined with the central and middle laterals, while in the dorsal valve they are 

 united with the middle lateral muscles. The scars of the anterior lateral muscles (j) of the 

 ventral valve are usually so closely united with those of the transmedians (i) that it is only in 

 unusually well-preserved specimens that the two scars can be differentiated; in the dorsal 

 valve they are more or less in advance of the centrals, and divided by a narrow median 

 septum or ridge ; they may be situated a little back of the center of the valve or at any distance 

 between that and a short distance from the frontal margm of the shell. In both valves the 

 scar of the parietal band closely circumscribes all muscle scars externally, crossing the main 

 vascular sinuses and terminatmg at the base of the flexure line of the area. 



The principal forms of Lingulella may be grouped mider five divisions by the outlines of 

 the ventral valve. 



Acuminate: Ovate-elongate: 

 Lingulella perattermata (PL XXI). Lingulella hella (PI. XIX). 



L. acutangula (PL XVII). Ovate-quadrangular: 

 L. manticula (PL XX). Lingulella mosia (PL XVIII). 



L. nanno (PL XXIV). L. winona (PI. XVIII). 



Subacuminate: Sub triangular: 

 Lingulella davisi (PI. XXXI). Lingulella auga (PL XXIV). 



Intermediate forms are illustrated by Lingulella martinensis (PL XXXVIII), which is 

 rounded subtriangular and acuminate, and L. pJiaon (PI. XXVI), which is both acuminate and 

 subacuminate. 



The outline, relative size, and position of the visceral areas (splanchnocceles) are exceed- 

 ingly variable. 



In Lingulella oweni (PL XVIII) the area is very short, while in L. acutangula (PL XVII), 

 L. perattenuata and L. similis (PL XXI), L. punctata (PL XX), and L. radula (PL XLV) it is 

 elongate. The narrow type of visceral area in the dorsal valve is seen in Lingulella acutangula 

 (PL XVII), L. Jiayesi (PL XXV), L. similis (PL XXI), L. manticula (PL XX), L. leos (PL XXIV), 

 L. ampla (PL XXVIII), and L. radula (PL XLV). 



The surface characters vary from the almost smooth and highly polished Lingulella nanno 

 (PL XXIV), on which only the faintest traces of radiating striae and depressed rounded lines 

 of growth are visible, to shells with strong concentric strise and lines of growth, Lingulella auga 

 (PL XXIV). Whenever the surface of the shell is sufficiently well preserved to permit close 

 examination radiating strise have been observed. 



Ohservations. — Prior to the investigation of the material described and illustrated in this 

 monograph,'' the data relating to Lingulella were too meager to permit detailed comparison with 



a Announced in a preliminary paper: Proo. U. S. Nat. Mus., vol. 21, 1898, p. 390. 



