446 CAMBRIAN BRACHIOPODA. 



Little is to be said of the vessels. It is the easiest, especially in the small shells, to distinguish the peripheral 

 canals, which lie on the plate-shaped border and are usually perceptible under the lens. It is more difficult to per- 

 ceive the principal vessels. They easily lose themselves in the small shell in the terrace of the plate-shaped border 

 and are but seldom perceptible in the large shell in spite of their freer position. 



The lateral vessels sink in their minuteness to microscopic sizes and are distinguishable only in exceptional cases 

 by use of strong enlargement. This is particularly the case in the ramifications, which are very indefinite in this 

 species. The figured specimens allow the vessel system to be perceived very faintly under the lens. 



As a rule the parietal band is seen, and it is also faintly suggested in the figures. In the small shell it also stands 

 out on account of the fine-grained character of the splanchnocoelic part of the shell. 



Formation and locality. « — ^Upper Cambrian: Obolus sandstone at the following localities: (395) At Joa, near 

 Jegelecht, 12 miles (19.3 km.) east of Reval; (395a) at Tihala, near Jegelecht; (395b) at Ilgast; (395o) at Asserien, 75 

 miles {121 Im.) east of Reval; (395d) at Ontika, about 95 miles (153 km.) east of Reval and 30 miles (48 km.) west 

 of Narwa; and (395e) at Isenhof, 85 miles (137 km.) east of Reval; all in the Government of Esthonia, Russia. 



Obolus conglomerate at the following localities: (395g) At Joa, near Jegelecht, 12 miles (19.3 km.) east of Reval; 

 (395h) at Tihala; (3951) at Ilgast; (395j) at Asserien, 75 miles (121 km.) east of Reval; (395k) at Ontika, about 95 miles 

 (153 km.) east of Reval and 30 miles (48 km.) west of Narwa; (3951) at Isenhof, 85 miles (137 km.) east of Reval; all in 

 the Government of Esthonia, Russia. 



Sandstone below the Obolus conglomerate at the following localities: (395m) At Joa, near Jegelecht, 12 miles 

 <19.3 km.) east of Reval; (395n) at Tihala; (395o) at Ilgast; (395p) at Asserien, 75 miles (121 km.) east of Reval; (395q) 

 at Ontika, about 95 miles (153 km.) east of Reval and 30 miles (48 km.) west of Narwa; and (395r) at Isenhof, 85 miles 

 (137 km.) east of Reval; all in the Government of Esthonia, Russia. 



(336o) Ungulite grit at Koporje, Government of St. Petersbui-g; and (336p) Ungulite grit at Eunitz, Government 

 of Pskow; both in Russia. 



Schmidtia conglomerate (part of the Obolus conglomerate) at the following localities: (395s) At Asserien, 75 miles 

 (121 km.) east of Reval; (395t) at Ontika, about 95 miles (153 km.) east of Reval and 30 miles (48 km.) west of Narwa; 

 and (395u) at Isenhof, 85 miles (137 km.) east of Reval; all in the Government of Esthonia, Russia. 



(321d) Drift bowlder of Obolus sandstone. No. 21 [Wiman, 1902, p. 68], on Fanton Island, parish of Borstil, 

 Province of Stockholm, Sweden. 



Obolus (Schmidtia) crassus Mickwitz. 



Plate XIV, figures 4, 4a-d. 



Obolus (Schmidtia) crassus Mickwitz, 1896, M6m. Acad. imp. sci. St.-P6tersbourg, 8th ser., vol. 4, No. 2, pp. 187-193, 

 PL II, figs. 47^9, 52-55. (Described and discussed in German as a new species; see below for translation.) 



Obolus {Schmidtia) crassus angulatus Mickwitz, 1896, idem, pp. 193-194, PI. II, figs. 50 and 51. (Described and 

 discussed in German as a new variety.) 



The original description by Mickwitz follows : 



Obolus crassus is the most abundant species of the subgenus Schmidtia. Its shells are massively built and to this 

 circumstance we are indebted for the remarkably strong development of all the internal characters, a development 

 which has been of the greatest importance in rendering possible the determination of the organization of the genus 

 Obolus Eichwald. With the large choice of well-preserved shells, specimens are found which preserve the finest detail, 

 as, for example, the outwardly and inwardly radiating accessory vessels, the opening of the principal vessels of the 

 large shell into the peripheral canal, etc. This development of the inner characters, standing alone among all the 

 Obolus species, which, so to speak, forms the basis of the anatomical part of the present work, and in many doubtful 

 cases has proved decisive, was the cause of directing to the shells of 0. crassus especial attention, which finds expression 

 in the increased number of figures and measurements. 



Diagnosis: Shells small, thick. Large shell (ventral valve) strongly arched, small shell (dorsal valve) less so. 

 Outline of shells broadly oval, umbonal borders convex. Surface of the shell faintly polished; concentric striation 

 rather fine, not uniform. Growth lamellse bordered by quite deeply engraved lines, toward the anterior border not 

 strongly projecting. Shell borders massive, blunt; plate-border formation slightly developed. Anterior border 

 and tip of the beak of the large shell mostly receding a little from the plane of the lateral border; in the small shell 

 projecting. Area very large, triangular, unevenly striate. In the large shell somewhat broken in the peduncular 

 groove; in the small one slightly deepened in the splanchnocoelic part; in both slightly excavated into furrows at the 

 almost rectilinear base. Peduncular groove deeply cut, narrowing slightly toward the tip of the beak. Traces of 

 the pseudo-area strongly developed, rectilinear, more or less converging posteriorly, separating the lateral borders. 



Thickening of the shell quite regular, somewhat stronger at the splanchnocoelic part. Septa of both hardly per- 

 ceptible. Corneous processes of the small shell suppressed by the strongly developed median swelling, which includes 

 the entire protruded splanchnoccele and fills the sinus. The median swelling has a fine median furrow at the anterior 

 border. Central groove of the large shell as a rule deeply sunken and bordered by swellings. Posterior part very 

 broad, anterior projecting tip parallel-bordered. 



a Locality 395 is represented in the collections of the United States National Museum; the remaining localities are taken from the references 

 given in the synonymy. The authority tor each locality cited is given in the list of localities, pp. 161-291. 



