210 BULLETIN 11, UNITED STATES NATIONAL MUSEUM. 



description are extremely shallow and frequently are not present 

 at all. 



The delicate parasitic growth and the thin-walled, polygonal, fre- 

 quently hexagonal, zooecia are so distinctive of this form that there 

 is little likelihood of confusing it with other species. 



Occurrence. — Kather rare in the Platteville limestone of Black River 

 age in Minnesota, Wisconsin, and Illinois. Very common in the 

 Kuckers shale (C2), Baron Toll's estate, near Jewe, Esthonia. The 

 incrusted shells of the latter locality occur mainly in the thin limestone 

 layers, which, where broken, show the Hyolithes and the fractured 

 zoarium as shown in figure 112 c. 



Plesiotypes.—Ca,t. No. 57293, U.S.N.M. 



Specimens and thin section from Kuckers shale. Baron Toll's 

 estate, in the collections of the British Museum. 



Genus STIGMATELLA Ulrieh and Bassler. 



Stigmatella Ulrich and Bassler, Smiths. MIbc, Coll., vol. 47, 1904, pp. 24, 33. — 

 Bassler, Bull. 292, U. S. Geol. Suxv., 1906, p. 27.— Cumings, Thirty-second 

 Ann. Rep. Dep. Geol. Nat. Rea. Indiana, 1907, p. 756. 



This genus was established for a group of the Heterotrypidse differ- 

 ing from other divisions of the family in having the walls of the zooe- 

 cial tubes noticeably thickened at periodic intervals, and in develop- 

 ing acanthopores only in these zones of thick walls. This periodic 

 thickening of the zooecial walls with an accompanying accelerated 

 development of the acanthopores has been found characteristic of a 

 number of Orodovician species which range in growth from the 

 incrusting to irregularly massive and ramose. In all other respects 

 these species are typical Heterotrypidae. The generic name was 

 selected for the reason that many of the species have unusually 

 distinct maculae or ''spots" composed of mesopores distributed over 

 the zoarium at regular intervals. Thin sections are almost a neces- 

 sity for the initial determination of a species of Stigmatella, although 

 additional specimens of a species can readily be detected after the 

 first example has been sectioned. The areas of thickened walls and 

 numerous acanthopores undoubtedly represent repeated mature 

 zones in the zoarium, for it is here only that mature zooecial characters 

 can be observed. Mesopores may be few, indeed absent, or numer- 

 ous. Without sections an incrusting species without mesopores 

 would be confused with a Leptotrypa like L, Jiexagonalis figured on 

 page 208, but a single layer of the Stigmatella would show several 

 alternately thin and thick walled areas. Comparison with other 

 genera of the Heterotrypidse might be made, but in every case the 

 distinct zones of Stigmatella remain the most diagnostic feature. 

 Another characteristic is the sparse development of diaphragms, in 

 which feature the genus approaches Delcayia, although differing in its 

 numerous mesopores. The following species include one with crenu- 



