154 PROCEEDINGS OF THE NATIONAL MUSEUM. 



similar British species, Bei/richia arcuafa (Bean) Jones and Kirkby, 

 the larue unterioi- lobe seems never to be divided, but in certain other 

 forms found in the Carboniferous rocks of England, notably Bey- 

 rlchiuf((stl(/!(d(( -Tones and Kirkby and B.fodicata Jones and Kirkby, 

 the lateral sulci ai-e deeper, and in the latter of the two species men- 

 tioned the separation and definition of the dorsal lobes reached an 

 extreme stage. Perhaps an even greater degree of contour differentia- 

 tion is shown in Jones and Kirk])v's Bet/ricJua tuhercalosjdno.sa^ in wliich 

 the ventral swelling that is usualh' a conspicuous feature in these 

 Carboniferous Beyrichiidj« is })roken up into three sharply defined 

 nodes. 



These various modifications of surface contour often remind strongly 

 of very diverse earlier Paleozoic generic types. The resemblance of 

 the simplest forms to PriniUla has })een mentioned already. Those 

 in the next stage, like Beyrichia arcuata (as figured by Jones and 

 Kirkby), B. craterlgera Brady, Beyi'lchlojh^lx .shajilex Jones and 

 Kirkb3', and BeyricMella cjregaria Ulrich and Bassler, are strikingly 

 like Klmdenia; and the seeming importance of this resemblance in 

 lobation is heightened by the fact that both have similarly unequal 

 valves. Fortunately, how^ever, in the true Klcedenitt^ it is the right 

 valve that overlaps the ventral edge of the left, while in these Car- 

 boniferous forms the opposite is the case. Another type, described 

 here as BeyriclueUa hoUiafornils^ is singularly like certain Ordovician 

 and Silurian species of BolUa (e. g., B. i?umila Ulrich); but, like the 

 other Carboniferous species under consideration, this has unequal 

 valves, a condition that does not occur in a true Bollia. Further, 

 while in the earlier Paleozoic Bolliae the diagnostic ''loop" is a very 

 constant feature, comparisons of numerous examples of the Beyriehi- 

 ella show that the loop which connects the nodes in this species is most 

 varia])le and in some instances is not to be distinguished at all. 



How to explain these seeming diverse alliances is no easy matter. 

 Still it is believed the explanation lies in the fact that the Beyrichiidte 

 had entered the period of their extinction in Carboniferous times. 

 Under such conditions it is reasona])le to assume that the type was 

 undergoing degeneration, and that this took place in the usual manner. 

 It may be suggested, therefore, that the resemblances to earlier stages 

 in the develoj^ment of the family above noted were occasioned by 

 reversion or ])y arrested development, whereby former immature 

 stages were retained through the adult stages, and thus became the 

 permanent specific and perhaps generic characteristics of the respective 

 types. Whether all the inecjuivalved Carboniferous Beyrichiid5\? were 

 derived from one or two survivals of the earlier types of the family, 

 or whether the suggested reversions affected similarly man}' such sur- 

 vivals, is a point that it may not be possible to determine satisfactoi-ily. 

 Still it is believed that careful comparisons l)etween the Devonian and 

 Carbouiferous j-(»])i'es(Mitatives of th(^ family will throw much light upon 



