no. 1646. REVISION OF BEYRICHIID2E—ULRICH AND BASSLER. 289 



The U. biicli'unui group is characterized by its comparatively long 

 shells, hut passes by easy gradation into the />. klasdeni group, the 

 mere proportionate shortening of the valves sufficing to bring about 

 some of the differences between them. On the other side it grades by 

 thickening and division of lobes into the B. tuberculata section. B. 

 lain ns'ts Kiesow is regarded as an extreme member of the group, 

 differing from the other species in the dissection of the anterior lobe 

 and its separation from the median lobe. 



GROUP OF B. TUBERCULATA. 



This section of the genus comprises a number of species and varie- 

 ties in which the anterior and posterior lobes are broken up by minor 

 furrows, the posterior lobe into two, the anterior lobe into from two 

 to six or seven node-like parts. Reuter's B. buchiano-tuberculata 

 would represent about the simplest type and B. jnist,il<>s<i Hall and 

 B. ncetlingi Renter the most complex. The anterior lobe is, as a rule, 

 larger than in other groups, and when not too much dissected, retains 

 the " leg-of-mutton shape " pertaining to this lobe in the majority 

 of the species of the genus. In the most simple species of the group 

 the posterior lobe is sharply constricted about its midlength, or some- 

 what above this point, the upper bulb being usually considerably 

 smaller than the lower. The anterior lobe in these is crossed obliquely 

 by a single curved furrow dividing the vertical upper part from the 

 much larger ventral portion. In the next stage of dissection (as. for 

 instance, //. tuberculata and B. bronni) the posterior lobe is usually 

 completely divided, while the anterior lobe is crossed by two parallel 

 furrows instead of one. In further stages the lower and largest of 

 the three divisions of the anterior lobe is broken up into a series of 

 three nodes and the middle division commonly into two, while the 

 upper may also be divided into two much smaller tubercles. 



Except in the most simple species, which of course are not far 

 removed from B. klasdeni and B. hue/liana, none of the Lobes are con- 

 nected ventrallv. This fact sets the group somewhat apart from the 

 majority of the remaining species here referred to the genus, and 

 allies it to the subgenus Steusloffia. A coarsely granulose surface 

 ornament of the lobes usually obtains except in the mosi highly dis- 

 sected specie-. As a rule, the main lobes are well separated and the 

 Carapaces large, thus differing from the otherwise similar group of 

 II. salteriana. 



Respecting the derivation of this group of species, the problem 

 seems at first sight very obscure. However, on closer analysis of the 

 lobes and comparison with Ordovician genera, the possibility of its 

 having sprung from Drepanella becomes more and more evident, so 

 that finally the idea assumes the rank of high probability and needs 

 but the discovery of one or two links to make it a certainty. To illus- 

 Proc. N. M. vol. xxxv— 08 -19 



