1916. No. 10. 



THE LOWER CAMBRIAN HOLMIA FAUXA. 



79 



compressed and shortened, whereby the breadth of the cranidium has become 

 disproportionateh' great in proportion to the length. 



Long- eyed and short- eyed forms. As pointed out in the descrip- 

 tion, this genus appears to comprise two different forms, one long-eyed, 

 the other short-eyed. The cranidium of the latter is probably somewhat 

 longer in proportion to the breadth than in the long-eyed form. This fea- 

 ture, however, is uncertain, as the the cranidium of this large form would 

 readily be deformed in various directions by pressure in the shale. 



It is probable that we have before us a sex dimorphism, as has been 

 surmised for similar conditions in other trilobites. 



Articulation of the thoracic segments. 



On account of the considerable size attained by this form the articula- 

 tion ot the thoracic segments is much more distinct than in the little form 

 Holmia Kjerulfi. 



Fig. 14. Articulation apparatus of Kjeridfia lata nov. gen. & sp. a. Two thoracic segments 



showing indentation in the postside of the dorsal furrow 'articulation cavitj-) and the bud 



like process (articulation processl that falls into the same X 2. b. Cavity and process of 



articulation somewhat dislocated X 4- 



As mentioned in the general description, we find on the thoracic seg- 

 ments a marked and deep incision in the posterior edge of the dorsal 

 furrow. Into this incision there fits a bud-hke process on the succeeding 

 segment. This remarkable feature is clearly seen in fig. 4, pi. XII, and 

 fig. 2, pi. XIlI. and is shown somewhat greatly enlarged in text-figure 14. 



Exactly the same structure is found in Holmia Kjcrtdfi Lnrs. 



Curiously enough, this does not appear to have been observed before 

 although Holm mentions the incision on the posterior edge of the segments. 



As the posterior part of each segment lies over the surface of articu- 

 lation of the succeeding segment, and the said process falls into the incision 

 of the dorsal furrow on the posterior side of the segment, a fairly firm 

 connection is established between the segments, and lateral displacement is 

 impossible. We have thus a primitive articulation apparatus that does not 

 appear to be found again in the later trilobites or in recent crustaceans. 



