KONGL. sv. VET. AKADEMIENS HANDLINGAR. BAND 34. \:o 8. L9 



for instance Olenus, straight and curved ridges occur in the various species, and con 

 sequently the form of this sort of ridge cannot be used as a generic character. Ii more 

 over differs from the ridge in the Olenellidse and the Paradoxidae in having the posterior 

 extreraity widened as a tubercle or node, which commonly has been called the palpebral 

 lobe», while in the older groups the ridge is only al the most a little pointed in both 

 ends or of equal thickness. 



Some persistant archaic genera as Conocon phe and Elyx inform us how this ridge 

 has originated and how widely different it is from that in the Olenellida-. ( >n the surfat :e 

 of their head shield no ridge is visible, but on the interiör side of i) wc Ixmold, as the 

 fignres show (PI. VI, tigs. 43 & 44) on each side of the glabella a ramifying system as 

 of the most minute vessels, which spread and cover the whole surface of the cheeks. 

 This reticulatiön issues from a main trunk that goes nearly straight in an oblique direc- 

 tion from the foremost segment of the glabella and emits narrow branches forwards and 

 backwards on its both sides. And these branches go on dividing till they occupy the 

 entire interiör surface of the fixed cheeks. The fignres representing this are taken from 

 Elyx laticeps fig. 43, and an almost similar i mage has been obtained from a species of 

 Conocoryphe fig. 44. 



Now the question may be asked, what does this network of branching canals mean? 

 I think we cannot gain a more plausible answer than that given us through the in- 

 spection of the soft parts that lie hidden behind the glabella of Limulus and its fixed 

 cheeks, the only living crustacean which offers the greatest homologies with the head 

 shield of the trilobites. Next below the shell of the glabella and the cheeks of Limulus 

 there is a complicated stratum of muscles and behind this the heart and the great central 

 circulatory system spreading from that centrum towards the sides of the head shield, the 

 vessels being the more fine and minute, the more they are elongated from the centre. 

 I now suppose that likewise in these trilobites the narrow and prominent glabella has 

 been the receptacle not only of a strong mäss of muscles, but also for the central part 

 of the circulation. This centre has there sent out two relatively strong ducts or canals, 

 one on each side of the glabella feeding all soft tissues inside and near the cheeks and 

 probably also other important parts of the body. It is also clearly seen in Elyx how 

 the orifice of the main trunc opens in the hollow of the glabellar apex. 



In all crustaceans, as far as is now known, their more or less härd calcareous or chi- 

 tinous skeleton is moulded by the subjacent tissues and glands. The sculpture of the surface 

 consequently is an outcome of the fashioning procedures showing what has been going on 

 below and what is still going on. If we then on the exteriör surface of the head shield of 

 the trilobites see the radiating lines in relief we must conclude that they are due to a 

 subjacent system of almost capillary vessels causing hollow impressions on the inside and 

 elevated ridges extcriorly. In Elyx the vessels have made no strong impression as to be 

 visible on the outside. In a couple of American Conocoryphse again (C. reticulata and 

 C. trilineata) the main trunc of the vessels has formed a short faintly elevated ridge. 

 In Solenopleura (Pl. VI, f. 45) the facial ridge has been fully developed, and by the casts 

 of the inside it is clearly seen that the main trunc of the vessels makes the inside of 

 the ridge and has been much incrassated. On the other hand the smaller branches issu- 



