I916. No. 10. THE LOWER CAMBRIAN HOLMIA FAUNA. 35 



Dimensions: I have measured many of tiie cranidiums, and the 

 results are given below in the accompanying table, which shows the 

 dimensions in mm. and also in parenthesis the proportions when the length 

 of the cranidium is put at I. The various figures seem to show a conside- 

 rable difference in breadth, but in some cases these differences are due to 

 the state of preservation. It must be observed that the various distances 

 are not measured along the curve of the cranidium but are projected 

 on a plane parrallel to the longitudinal axis of the cranidium. 



Thorax. Only one or two isolated fragmentary thoracic segments have 

 been found. Text-figure 6a shows one of them enlarged; we see two thoracic 

 segments, the posterior one being somewhat well preserved on the right 

 side. The axis has a comparatively broad appearance. The pleura has a 

 distinct, faint, oblique furrow, and the anterior raised portion extends as a 

 comparatively long, slightly bent pleural spine. Free cheeks and pygidia 

 were not found. 



Hypostom. In all probability a quite small ■ — only about 2mm. 

 long — hypostom belongs to this form. (PI. V, fig. 6.) It is somewhat 

 incomplete, the anterior wings being absent. It is elliptical, with prominent 

 arched oval body. The latter narrows markedly backwards, and is there 

 surrounded by a narrow, arched, almost half- moon shaped part (posterior 

 section). The anterior parts are lacking in the specimen, but the posterior 

 parts are clearly visible on one side ; They form a somewhat broad rim 

 that is cut off posteriorly in an abrupt wa}-. The maculae are not visible. 

 As the hypostom of the Ellipsocephalidae are but little known, this small 

 but unfortunately somewhat incomplete specimen is of considerable interest. 

 It appears to be specially characterised b}' the circumstance that the poste- 

 rior section of the central part of the Hypostom embraces the oval body 

 in a more arched wa}' than is usually the case, and that the posterior 

 wings are very prominent and tlath' extended. The last named character 

 in particular makes it diff'er from the hypostom oï Agraulos difformis Ang., 

 which was described by Brøgger ^ 



It is noteworthy that neither this species of Ellipsocephalidae nor any 

 of the others were found in complete specimens in the fine clastic 

 Holmia shales at Tomten, whilst Mesonacidae are often found whole. As 

 a matter of experience we more often find complete specimens of small 

 than of larger forms. This is probably due to the fact that the segments 

 of Ellipsocephalidae are more loosely attached to each other than is the 

 case with Holmia and Kjernlfia. In the two latter I have succeeded in 



* Paradoxidesskifrene ved Krekling (Nyt. Mag. f. Naturv. Vol. XXIV, pi. IV, fig. 7). 



