456 ME. S. S. BTJCKMAN ON THE BAJOCIAN [Aug. 1895, 



Genetic observations on brachiopoda indicate that biplicate 

 Terebratxdoe, are, at any rate in the Jurassic rocks, independent, or 

 heterogenetic, homoeomorphous derivatives of non-plicate forms. 

 The method of nomenclature hitherto adopted rather tends, however, 

 to confuse these biplicate forms together, either as the same species, 

 or as varieties of one another — and they are often difficult to 

 distinguish — and totally overlooks their relationship to the non- 

 plicate forms (from which they appear easily separable), thus 

 ignoring their genetic affinities altogether. A case in point is 

 Davidson's confusion of what is now named TJptoni with Terebra- 

 tula perovalis ; and another is the inclusion of T. Phillipsiana as a 

 variety of the later-appearing T. Phillipsi ; and, again, the placing 

 of T. submaxillata as the same species, or as a variety, of the much 

 later T. maxillata. A primary objection may be taken, that a 

 variety cannot live before the species of which it is supposed to be 

 a variety came into being — especially in a case where it is about 

 2 ' ages ' earlier ; and particularly when there are either no links 

 between the forms, or only casual, isolated examples — apparent but 

 not real links — homoeomorphous but not homogenetic forms. 



Really the course of development was this : there was a non- 

 plicate stock with a tendency to throw off a constant series of 

 plicate, and consequently more or less homoeomorphous, derivatives. 

 The first stage was a circular form ; the second an elongate form, 

 both non-plicate, or the latter feebly uniplicate ; the third an uni- 

 plicate or incipiently biplicate form ; the fourth a pronounced 

 biplicate form ; the fifth a reversion to an uniplicate form, with 

 pronounced raised fold. Or the second stage may be omitted, and 

 the circular form may be retained, becoming directly plicate ; or 

 the second form may be directly multiplicate — a development of the 

 non-plicate first form — as in Terebratula fimbria. 



Taking the punctata-siock, there is, first, in the Middle Lias, 

 the circular form, Jauberti, with derivatives — the thin, elliptical 

 punctata, the subcylindrical subpunctata, and the globose Edivardsi. 

 Later (Dumortierice hemera) appears haresfieldensis, called by David- 

 son a variety of punctata, but really a mutation of a punctata-\\ke 

 form ; and this becomes a starting-point for a series of new deve- 

 lopments : it breaks up into varieties, pointing, apparently, to 

 Terebratula perovalis (Murchisonce hemera) as a development in one 

 direction 1 ; while Terebratula ampla, J. Buckm., shows the per- 

 sistence of the circular form. Thus ampla, usually called a ' variety 

 of perovalis,' is, biologically, a less modified form, is nearer to the 

 original stock, and presumably had a somewhat different origin 

 from that of perovalis. Then, somewhat later than haresfieldensis, 

 namely, in the opalini hemera, lived a rather more circular form 

 with acute margins, for which no name has yet been discovered ; 

 but the form is found at Irocester (Gloucestershire), Chiselborough 



1 Perhaps through T. trilineata ; but the date of that fossil has not been 

 recorded with sufficient accuracy for the present purpose. 



