69 



Tliis, however, maj üvst bc divided into two parts. We sliall 

 first decide wliicli edge is to be regarded as hinge line, or in other 

 words along which edge the two valves moved with respect to each 

 other, and next try to find out which edge is to be considered as 

 anterior end, or in other words which of the t^vo short edges was 

 situated nearest the head of the animal. The first part of the question 

 will be discussed first being easiest to solve. 



M'CoY had got so accustomed to the position which was to be 

 given to the valves when they were still regarded as cephalic shields 

 of trilobites that, in mentioning the criteria of the genus Beyrichia, 

 he described the long curved edge as dorsal side and the straight 

 edge as ventral. He did this also in a later publication (9, p. 135), 

 though in the meantime Salter (10, p. 351) in the description of a 

 species of Bei/richia had taken the straight side as the dorsal edge. 

 Later investigators always followed Salter's example and in my 

 opinion this is quite in accordance with what we see elsewhere in nature 

 in similar cases, so that I agree with him and consider this part 

 of the question settled. 



Now the second part of the problem remains to be solved and 

 this will take us longer. 



In mentioning the criteria for the genus Beyrichia, m'Coy states 

 that the least developed end is to be regarded as anterior also 

 recognized by the location of the median furrow and the oval node 

 in it nearest to that end. There are no data given for this assertion. 



Nor does Jones (11, p. 85) mention any. Among the criteria of 

 the genus Beyrichia he states that the posterior end of the valves 

 is higher than the anterior end. From the description of Beyrichia 

 tuberculata, however, it appears that he regards the end with the 

 two nodes as the anterior end whereas this part is the highest. At 

 a later date Boll (12, p. 118) and Krause (13, p. 30) did the same. 



Barrande (14, p. 467) is the first who tries to prove that the 

 position of the Beyrichian shells suggested by Jones, is probably the 

 right one. He does this, however, by comparing the nodes at one 

 end of the shells of some Bohemian Ostracoda with the lobes of the 

 Tribolite-glabella. As these nodes occur on the tapei-ing part of the 

 shells, he takes this to be the cephalic part. As for myself, I would 

 not follow this line of evidence ; nor does it lead us any further 

 towards the orientation of the Beyricldan shells, as the latter have 

 nodes at both ends. Barrande himself admits this too. 



Contrary to all former writers Reuter (15, p. 625) assumed that 

 the end with the three bulb-like parts, was the anterior eiid. He 

 did this because measuring showed him that this very end is the 



