162 G. F. MATTHEW : ILLUSTEATIONS OF 



iu Europe ; and an examination of the above table will shew that a corresponding develop- 

 ment of the genus took place at the same horizon in the Cambrian beds of America. 



One of the oldest types of the genus Aguostus on this side of the Atlantic, and also 

 apparently iu Scandinavia was that indicated by Tullberg as the Regii or relatives of 

 Agnostus rex ; this form of Aguostus is characterised by an enlargement of the front of the 

 glabella (Trans. Koy. Soc. Can., Vol. Ill, Plate VII, fig. la), which is an embryonic feature 

 of the trilobitcs of Groups 2 and 3, and a permanent feature of Paradoxides. The species 

 arranged by Tullberg in the group of Aguosti which he calls Parvifrontes are remarkable 

 for very short glabellas, and in one of our Acadian species, {A. tessella, loc. cit, fig. 7 a) it 

 will be observed that we have a transitional form with two lobes in the glabella, of which 

 the posterior represents the full glabella of a Parvifrons. The anterior lobe in this species 

 is only faintly outlined, but if it were as distinctly elevated as the rest of the glabella the 

 species would come into the group which Prof Tullberg designates Fallaces, or the relations 

 of Aguostus fallax. It is fitting that this species should first appear in Baud d. The 

 Longifrontes of Tullberg which possess a wealth of species in Scandinavia, will probably 

 be found richer in species in Acadia than they at present appear when the measures of 

 Baud d are more fully explored. Of the group Lœvigati, large and well marked species 

 are known in Europe aud one iu Newfoundland.' Iu this group the fusion of the glabella 

 into the body of the head-shield is carried to the highest pitch ; but the section belongs 

 to the highest part of the Paradoxides beds, and naturally has not been met with in the 

 measures represented in the table. ' 



Altogether the succession of the Agnosti iu the St. John Basin does not unfittingly 

 represent to us the development of the genus in geological time, for it gives, first, those 

 with a prominent glabella, and later, those with a glabella becoming obsolete. 



The genus Microdiscus is an advance of development over Aguostus in many res- 

 pects. Though some palœoutologists have classed them together, the differences are too 

 great and important to admit of such a connection. Microdiscus is not cast in the same 

 unchanging mould as Agnostus, but shews a far greater capacity to vary, both in the 

 number of joints in the thorax, and iu the pygidium, and in other respects. As originally 

 defined by Salter, it had four joints in the thorax and this number has been found to 

 characterise all the species whose thoraces are known except two ; one of these {M. 

 spedosus) was discovered by Mr. S. W. Ford in the Cambrian slates near Troy, New York, 

 and possesses three joints iu the thorax ; the other is Microdiscus Daivsoni, of which there 

 is an example three-quarters grown, haA^ing only two segments in this part of the body ; 

 judging from analogy in the species of trilobites in Groups 2 and 3 of our table, the adult 

 of this species would not possess a larger number of joints than one three-quarters grown, 

 and therefore this species would in respect of the thorax depart from all other known 

 Microdisci in the number of joints, and coincide with the normal number iu Aguostus. 

 M. Datvsoni with its two-jointed thorax is the oldest species of the genus of which the 

 thorax is known. 



Another point in which this genvis differs from Agnostus is in the variable number 

 of riugs in the axis of the pygidium, both as regards the species when compared with 

 each other and as relates to an increase of the number of rings in the individual, during 



' A species of this section of Agnostus has hitely been found in Div. 1 d in the St. John Ba.sin. 



