136 G. P. MATTHEW : ILLUSTEATIONS OP 



said to be spined, and in this he may have followed the indications of Linnarsson in 

 reference to L. costatus given aboA'e ; but I cannot find that L. aculeatus is represented in 

 any figure as possessed of spined cheeks, and my impression, therefore, is that these 

 savants, by this description of the angles of the cephalic shield in this genus, intended to 

 cover the characters of L. costatus and L. micropthalmus (the latter described by Angelin as 

 an Anomocare.) 



It would seem that some difficulty was experienced by Angelin's successors in find- 

 ing the type of this genus. Angelin's diagnoses are in many cases so very brief, that 

 without very perfect figures it would be a matter of difficulty to recognise his types. 

 Even those who have collected from the localities whence his species were obtained, have 

 not in all cases agreed as to the species intended by his descriptions, and L. aculeatus is 

 one in which the identification is not assured. Dr. Brogger refers to it a head-shield from 

 Stage 1 at Krekling, in Norway, which has a much longer spine on the occipital ring 

 than is represented in Angelin's figure of this species ; and the head differs in other 

 respects. Gr. Linnarsson subsequently asserted that this head is not that of Angelin's 

 species, but is a variety of another species, and himself figures several heads which he 

 declares to be the true heads of L. aculeatus. These head-shields of Linnarsson closely cor- 

 respond to those of the Acadian species L. Ouangondianus, Hartt, and occur at the corres- 

 ponding geological horizon in Sweden. If Linnarsson's determination is right, I infer that 

 L. Ouangondiamis is the American representative of Angelin's species. But this species 

 bears no spine at the genal angles, and, by the peculiar form of the free cheeks and the 

 hollow space in front of the glabella, while it corresponds to Angelin's figure and 

 description, it differs from numerous species referred by some authors to Liostracus and 

 by others to Ptychoparia. To avoid confusion, the generic name Liostracus should be 

 limited to such species as L. acideatus and the two species of the Acadian measures herein 

 referred to Liostracus. This genus then becomes characteristic of the lower part only of 

 the Paradoxides zone, and does not exhibit the wide vertical range of its fellow genus 

 Ptychoparia. 



The following is Angelin's original description of the genus : — 



" CorjMs longitudinaliter trilobum, crusta laevi tectum. 



"Caput immarginatum sulcoque intramarginali : anguli exteriores Frons ovata, 



integerrima. Oculi parvi, distantes, semilunati, genarum medium versus siti ; sutura 

 facialis postice ab oculis ad marginem apicalem ^ ducta, anticeque ad marginem apicalem 

 decurrens. 



" Tliorax 



"Abdomen rotundatum, rachi ^ costisque lateralibus distinctis." 



The following is proposed to fill in the ellipses in Angelin's description and to give a 

 more correct view of the form and size of the pygidium : — 



Corpus, elnngate-ovatum, anticâ latitudine extensu, longitudinaliter, etc. 



Caput immarginatum aut paulo-marginatum, sulcoque intramarginali; anguli exter- 

 iores rotundati, genœ mobiles longittidine liratce, excavatœ, parvis auriculis terminatis. Frons, etc. 



' Om Paradoxides skifrene vid Krekling, Tafl. iii. fig. 3. 

 '' Om faunan med Conocoryphe exsulens, Tafl. ii. flg. 12 to 15. 

 " Posterionm is probably intended rather than apicalem. 

 ' This does not describe the form in the American species. 



