no. 104(3. REVISION OF BEYRICHIIDJE—TJLRICH AND BASSLER. 319 



The genus is divisable into two sections, the first comprising the 

 greater part of the known species, among them the genotype. In this 

 group the posterior and median furrows are shorter than in the sec- 

 ond, and the anterior furrow either wanting or more or less well de- 

 veloped. In the second group, of which Beyrichia halli Jones (Bol- 

 lia halli Ulrich) is a good example, the posterior and anterior fur- 

 rows are long, subequal, and extending nearly or quite across the 

 valve. The general expression is rather strikingly dissimilar in the 

 two groups, but on close comparison it is found that the differences 

 are not essential and, moreover, that the extremes are bridged by 

 connecting links. 



The Klcedenella halli, as it should now be called, simulates Bollia 

 .s>/iniii( trica (Hall) to such a degree as to suggest its derivation from 

 that earlier Silurian (Rochester shale) species. The Bollia, however, 

 has not overlapping valves as has the Klcedenella. Besides, the lat- 

 ter type appears to have been established already in Clinton time, 

 indicating that its ancestors are to lie looked for among Ordovician 



61 



O 



Figs. 61-04. — 61. Left valve of Bollia symmetrica (Hall). 62. Right valve of 

 Klcedenella iialli (Jones i. Copied from Jones. 63. Right valve of Klcedenella 

 tirgida. new species. 04. right valve of beyrichia? parallela utirich, v -' 20. 

 (After Ulrich.) These figures illustrate (1) the similarity of expression 

 of Bollia to Klcedenella, and (2) the fossible derivation of Klcedenella from 

 the Richmond species Beyrichia? parallela. 



species. Such a possible ancestor is found in the peculiar Richmond 

 species described by Ulrich as Primitia or Beyrichia parallela. The 

 general form of the valves and the lobes are sufficiently like Klce- 

 denella to encourage the belief that it fulfills the requirements of 

 the case. It is to be regretted, however, that only separated valves 

 of this Richmond species are known, so that it is difficult, if not im- 

 possible, to decide the question by showing agreement also in the 

 matter of inequality of its valves. 



Because of the mentioned resemblance to Bollia, Ulrich in 1894 

 tentatively referred species of the K. halli section, including A'. 

 clarkei (Beyrichia clarkei Jones) and two new species, to Bollia. 

 Although this reference is now thought to have been in error, it must 

 be admitted that it is not easy to show just why the group was not 

 descended from Bollia regularis and B. symmetrica. At that time 

 the senior author had very few specimens of the type now discrim- 

 inated under the name Klcedenella, and A", halli and A'. (■/<//■/,■(! were 

 known to him only from the rather indefinite figures and descriptions 

 published by Jones, which neither showed nor mentioned the inequal- 



