PATERINID^. 331 



surface which is punctate and curiously ridged, very much Hke the outer surface in partly worn 

 shells. (See PI. VI, fig. Im, of the outer surface.) 



Substance of shell mostly calcium phosphate [Schmidt, 1888, p. 23]. The shell is thick in 

 small shells, b\it does not increase proportionately in thickness with age. 



The largest shell in the collection before me has a length of 32 mm., exclusive of extension 

 of apex over area ; width 30 mm. 



Ventral valve convex, with apex projecting a little over the posterior margin. A false area 

 is more or less definitely outlined ; in some specimens it is quite clear and in others very indefi- 

 nite ; it is much like some of the less clearly defined false areas of species of Micromitra. When 

 well defined the posterior margin of the valve arches slightly, as in Plate VI, figure le. 



Dorsal valve slightly convex, with a very low submarginal apex. The interior of the valve 

 is usually without traces of muscular or vascular markings. In one worn shell a median sinus 

 occurs, and what appears to be the path of advance of the anterolateral muscle scars (PI. VI, 

 fig. If) ; another unworn shell shows a trace of radiating lines near the median line ; and a third 

 has two plainly marked muscle scars a little in advance of the apex and some distance from the 

 median line ; these occupy the position of the posterior muscle scars as they occur in the dorsal 

 valve of ScMzocrania. 



Observations. — The preceding description is based on specimens in the collection of the 

 United States National Museum. Mr. Schmalensee, a collector in the Geological Survey of 

 Sweden, at my request collected a number of fine specimens for the. United States National 

 Museum. The museum has also received a number of casts from other sources. 



Schmidt [1888, p. 22] describes a hooked process as occurring in a cast of a dorsal valve. 

 After noting his expression of doubt as to whether the process belonged to the shell, and also 

 after a study of the interiors of finely preserved dorsal valves, I can not retain the view that 

 such a process exists in the dorsal valve of this species. We may attribute the apparent occur- 

 rence of it in the cast studied by Schmidt to the accidental attachment of some foreign body, 

 or it may be that the cast belongs to some other genus and species. 



There is no other species of the genus that is closely related to Mickwitzia monilifera; M. 

 formosa has the same general character of shell, but its ventral valve is more erect, much larger, 

 and it difi^ers a Httle in the details of the surfaces of the interior layers of the shell. 



Formation and locality. — Middle? Cambrian: (3111 [Wiman, 1902, p. 57]) Drift bowlder of bluish calcareous 

 sandstone, No. 31, on Biludden, about 20 miles (32.2 km.) east of Gefle, Province of Gefleborg; and (311u [Wiman, 

 1902, p. 57]) drift bowlder of blue calcareous sandstone, Soderarm lighthouse, near Norrtelje, Province of Stockholm; 

 both in Sweden. 



(311e) Drift bowlder of rusty sandstone, south of Lumparn, parish of Jomala; (311f) drift bowlder of gray quartz- 

 itic sandstone, at Slemmem, Mariehamn; (311c() drift bowlder of gray quartzitic sandstone, at Yttemaset, Mariehamn; 

 and (311r) drift bowlder of gray quartzitic sandstone, at Granboda, Lemland; all [Wiman, 1902, p. 57] on Aland Island, 

 Finland, Russia. 



Lower Cambrian: (390j) Eophyton sandstone at Lugnas, 8 miles (12 km.) south-southwest of Mariestad, Province 

 of Sharaborg; and (390k) Eophyton sandstone at Prestorp, north of the hill of Billingen, Province of Skaraborg; both 

 in Sweden. 



(396q) Upper boundary of the blue clay with the Fucoid sandstone, the "Mickwitzia conglomerate" at Reval; 

 (396r) in the "upper strata" at Streitberg, near Reval; (396s) in loose blocks on Kosch Brook near Likkat, near Reval; 

 (396t) at the sandstone capes of Ziegelskoppel, Kakkomaggi (also Rocca al Mare), near Reval; (396u) in loose blocks 

 on the lower course of Jaggowal Brook, about 20 miles (32.2 km.) east-southeast of Reval; (396v) at the upper boundary 

 of a dolomitic sandstone with which the Olenellus stratum begins, below the cement factory on Kunda Brook;" and 

 (396w) in the glauconitic sands above the main mass of the clay, below the cement factory on Kunda Brook; all [Schmidt, 

 1888, p. 24] in the Government of Esthonia, Russia. 



Mickwitzia ocoidens Walcott. 



Plate VI, figure 4. 



Mickwitzia ocoidens Walcott, 1908, Smithsonian Misc. Coll., vol. 8, No. 3, p. 54, PI. VII, fig. 1. (Described as below 

 as a new species. Fig. 1 is copied in this monograph, PI. VI, fig. 4.) 



Only crushed and broken specimens of this shell are available. One of these shows that 

 the apex of the ventral valve was a little above the posterior margin of the shell, very much as 



a Specimeris of this species from the locaUty on Kunda Brook are included in the collections of the United States National Museum. 



