90 JOHAN KIÆR. M.-N. Kl. 



the straight pleural furrow is the original one, the oblique furrow the 

 secondary one, and the more highly developed stage. We may, e. g. in 

 the Upper Cambrian Olcnidae see that the oldest, primitive genus Olenns 

 has still straight furrows, the later species oblique ones; the true Paradoxidae 

 may therefore be assumed to have developed from forms with straight 

 furrows, a theory that is supported by the earliest stages of development. 

 That which must therefore be specially emphasized is the circumstance that 

 as far as this structural feature is concerned there is a sharp line of demarca- 

 tion between the two families. No stage of transition exists. 



Even greater importance, however, must be attached to the difference 

 shown by the sculpture of the shell In Mesonacidae we find a t3'pical 

 somewhat fine or coarse net sculpture; only two or three species of the 

 genus Olenellus exhibit an undoubted divergence from this, the sculpture 

 (on the glabella) being transformed into fine anastomosing lines {O.freinonti 

 Walcott) or they are granulated. In Paradoxides on the other hand we 

 never find net sculpture; the shell is either smooth or most commonly 

 marked with fine granulae and fine raised lines. In this respect also there 

 is a distinct line of demarcation between the two families, and it must be 

 specially emphasized that the genus IVauiicria, from which Walcott desires 

 to derive Paradoxidae has a net sculpture of a specially typical and marked 

 character. It would be very remarkable if none of the Paradoxidae has 

 preserved anything of this constant feature, if their phylogenetic evolution 

 has proceeded as contended by Walcott. 



In conclusion we must point out the important circumstance that even 

 the development of the young stages in Paradoxidae does not indicate such a 

 descent. It is not as well known as that of Mesonacidae but is nevertheless 

 sufficiently well known as far as concerns one or two Bohemian species 

 that we can make a comparison. Of special importance is the line of 

 development that P.E. Raymond^ with great probability demonstrated in 

 the case of Paradoxides nignlosus Corda (Hydrocephalus saturnoides, Par. 

 orphaniis, Par. pusillus, Par. rugulosus). He also describes a Prostaspis 

 stage which, however, probably belongs to another species. I reproduce 

 his summary respecting ontogeny. 



"From the above survey of the material now available for the study 

 of the ontogeny of Paradoxides, we see that the youngest shell or protaspis 

 is very similar to that of Olenellus. The glabella in the youngest speci- 

 mens of both species of Hydrocephalus is specialized and unlike that of 

 any other trilobite of which the young is known, in that it occupies a large 



1 Notes on the Ontogeny of Parado.xidcs etc. pag. 234. (Bull. Mus. Comp. Zool. at 

 Harvard College, Vol. LVIII, 1914, No. 4 ) 



