KONGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 34. \:<> 8. II 



sence. Nor are there any Bigns of closed up Butures, which also could nol possibl} be 

 expected in so early a stage of evolution. It m.i\ then be taken as well Bettled thal a 

 fundamental character in these the oldest 1 of all known trilobites is the total wanl of a 

 facial suture and a compactness of the whole head shield which låter i- broken up in 

 several parts through the disjunction of the free checks. In the Lower Silurian forma- 

 tion there are a few genera sharing in the same structure of the head shield, though 

 l>v no means else related. Such are Dindymene, Areia, Carmon and in the U. Silu- 

 rian Cromus. The two species forming Barrande's genus Dindymene are so dissimilar 

 that Dind. Friderici Augusti had better t" be removed to a neu genus and the firsl >\(- 

 scribed one to be retained as type of the genus Dindymene. The same i» the case with 

 Carmon, where the type species C. mutilus is blind and without free cheeks while the 

 other species, known only by its fixed cheek-» and glabella is one of the Olenidae. 



II. Blind trilobites with facial ridge. 



This large division embraces the second and third groups or, with a few exceptions, 

 all the rest of the Gambrian trilobites on account of a feature in the cephalic sculpture 

 common to theni all, though widely different as to its first origin in both. Whar forms 

 the prominent and common characteristic of the>e two groups is the presence of the facial 

 ridge, which emanates from the basis or the front of the foremost segment of the glabella 

 and in a great variety of different shapes continues backwards near to the posterior börder of 

 the head. It has reeeived several names as eye-line, palpebral lobe, ocular ridge, eye-lobe, 

 ocular Jillet (Matthew). In German it is named Augen-leiste, in French filet (Barrande) 

 and in Swedish ögonlist. 1 Some authors make a difference between the more narrow 

 part, calling it eye line, and the thicker posterior node, which they na me the palpebral 

 lobe proper. 



A- this peculiar ridge exists before any facial suture has made its appearance 

 and separated the head shield in tive parts, viz. the median glabellar part, the two tixed 

 cheeks and the two free cheeks, and as it occurs in genera which never possessed any facial 

 suture, and where no eye ever was formed, it is not adequate to call it an ocular ridge etc. 

 the more so, as it, at least during a long series of genera succeedinff each other, has had 

 no connection whatever with any eye. I therefore propose to call it facial ridgt (in swe- 

 dish faciallist). It occurs on the head of almost all Cambrian trilobites, excepting the 

 archaic ones, and it is retained in the låter Cambrian Peltura, Sphaerophthalmus etc, which 

 have real, compound eyes, as well as in a few Lower Silurian genera as Triarthrus, Plio- 

 mera, 3 Euloma, in the Upper Silurian Arethusina and Acidaspis and most persisting in 



1 Oldest in that sense that they are the descendants of an archaic praecambrian stock, the chief charac- 

 teristics of which they have retained in the main unchanged and persisting long ages after the close of the 

 Cambrian times, some, as Agnostus. continning high up in the Lower Silurian. 



- That name is the most current araongst the swedish authors, together with tpalpebrallob : Holm 

 saya ögonlob and frontallob. 



3 Pl. Törnquisti Holm. 



