542 CAMBRIAN BRACHIOPODA. 



showed, together with some suggestions of the last-mentioned genus [Linguld], unmistakable marks of the'genus Obolus, 

 so that the species, which would not be assigned to any of the other groups, had to be ranked in a special subgenus of 

 Eichwald's genus. 



The internal characteristics of the Cambrian and Silurian Lingulae have not hitherto been established so com- 

 pletely as to render it possible to compare the organization of this extinct group in detail with tbe recent Lingula. 

 It is probable that more accurate knowledge of these oldest Lingulse will lead to a change in the generic designation. 

 A similar uncertainty of coui'se exists also in regard to the relations of the genus Obolus to the above-mentioned Lin- 

 gulse, and if it be possible to ascertain the internal features of the latter to the same extent as in the genus Obolvs, 

 0. lingulseformis will probably become the connecting link between these old extinct genera. 



/ LlNGULELLA (LePTEMBOLON) LINGULSEFORMIS (Mickwitz) . 



Plate XIV, figures 5, 5a-b. 



Oholus (Leptembolon) lingulsformis Mickwitz, 1896, M^m. Acad. imp. sci. St.-Petersbourg, 8th ser., vol. 4, No. 2, 

 pp. 200-204, PI. Ill, figs. 10-17. (Described and discussed in German ss a new species; see below for trans- 

 lation. Figs. 17a-b are copied in this monogi-aph, PI. XIV, fig. 5.) 



Oholus {Leptembolon) lingulxformis solidits Mickwitz, 1896, idem, pp. 204-20-5, PL III, figs. 18 and 19. (Described 

 and discussed in German as a new variety.) 



The original description by Mickwitz follows: 



Shells moderately large, quite flatly arched. Outline pointedly oval to subtriangular. Beak and posterior 

 part of shell laterally somewhat flattened, forming two slightly pronounced roundish edges, which converge to the 

 tip of the beak. Anterior part of shell similarly flattened toward the anterior border. 



Surface of shell very smooth, very lustrous, like varnish. Concentric striae very fine and regular, especially 

 sharp cut at the posterior borders. Growth lamellss indistinctly bordered. In age, clearly distinguishable by the 

 somewhat projecting borders of the lamellae. Anterior and lateral borders very thin, sharp-edged, brittle, lying 

 in the plane with the uncovered umbonal borders. Posterior part of the shell scarcely thickened, area more in shape 

 of covering of the hollow tip of the beak. Pleurocoelic part of the area somewhat prolonged into the lateral borders. 

 Peduncular groove very broad, somewhat diminished toward the tip of the beak, lying deep but only flatly arched 

 on account of the deficient thickening of the beak. Traces of the pseudo-area rectilinear, converging to the tip of 

 the beak. 



Splanchnoccele of the large shell (ventral valve) drawn forth, anterior point of the same bordered in the shape 

 of a bow, and somewhat projecting. Terrace of the thickening of the posterior part of the shell diverging into the 

 brachiocoele, longitudinally palmately striate. Central groove of large shell very flat, posterior part indefinitely bor- 

 dered, tip drawn out, parallel bordered and anteriorly semicircularly rounded. Corneous processes of the small shell 

 (dorsal valve) strongly receding, sinus indefinitely flattened. Median swelling of small shell strongly developed, 

 beginning in the sinus as a thin, roundish ridge, gradually swelling, reclining beyond the places of attachment of the 

 anterior lateral muscles, and ending shortly before the anterior border in the shape of a pestle. Principal vessels 

 distinguishable in their beginnings, not traced in the brachiocoele. Peripheral canals and accessory vasculai- furrows 

 in both shells only distinguishable in indistinct traces. Places of attachment of the umbonal muscles large, longi- 

 tudinally striate, not to be separated from the place of attachment of the peduncular muscle in the large shell. Places 

 of attachment of the central and outside lateral muscles to both sides of the central gi-oove of the large shell, separated 

 from each other and from those of the middle lateral muscles. The latter lie before the semicircularly bordered apex 

 of the central groove. Central muscles (with their advance corresponding to the growth of the animal) leaving behind, 

 in the small shell, a row of traces, which converge into the sinus. Places of attachment of the combined outside and 

 middle lateral muscles, as also those of the transmedian muscles of the small shell, very near the border of the shell. 



Observations: The shells of 0. lingulieformis show a different form of outline in their youth than in their age. 

 For the species of the subgenus Euobolus, we have demonstrated a proportional growth, and the same also takes place 

 in the species of the other subgenera, as the consideration of the concentric striae teaches, which marks the stages of 

 age. The form of outline remains the same, except in the very first stages of youth. In 0. lingulxformis the outline 

 of the shell is elliptic until in old age, and then first begins to become subtriangular through the growth lamellae pro- 

 jecting mostly at the anterior border and anterior lateral borders. Self-evidently, the separation of the substance 

 of the shell takes place also in this stage along the entire border of the shell (including the area), but the lamellae run, 

 so far as theii- external part (which fonns the sm-face of the shell) comes into consideration, so sharply into the lateral 

 borders that they do not help to broaden the posterior borders of the shells, but only to thicken them vertically. The 

 outline of the posterior part of the shell, therefore, remains the same, while the anterior part steadily gains in breadth. 



In connection with this manner of formation stand the extraordinarily sharply sculptured concentric striae at 

 the lateral borders of the shell, which are closely crowded against each other. 



This characteristically subtriangular outline, which, in connection with the flat arching of the shell, marks the 

 typical form of 0. lingulseformis, is, however, in some specimens given to essential deviations. The beak becomes 



