54 JOHAN KIÆR. M.-N. Kl. 



Finally it also appears that Protoleiitts ^ might be connected with 

 pimitive forms of the Stremiella group. 



I will content myself with these brief indications, m}' material not 

 inviting further investigation of these difficult questions, the solution of 

 which would demand a very comprehensive study of the phylogeni of 

 the whole Olenid tribe. 



In any case, it seems clear from what I have here developed, that the 

 closely related genera Streniidla and Ellipsoaplialits belong to an ancient 

 and somewhat large form-group of small trilobite forms, which played a 

 prominent part in the Lower Cambrian and the oldest Middle Cambrian 

 fauna, a form-group from which the younger Middle Cambrian Ellipsocc- 

 phalits forms, and possibly also the Anouocair- forms, mav be derived. 



Fam. Mesonacidae Walcott. 

 Holmia Matthew. 



This genus is distinguished by the following characteristics: Crani- 

 (limn broad, semicircular, whilst the thoracic segments are comparatively 

 narrow. Glabella extends anteriorly and is here broadly rounded. The 

 second pair of lobes grow broader outwards and are slightly geniculated. 

 Both the occipital segment and the axis of the thoracic segments aie divided 

 by a distinct transverse furrow. The eyes are large, with markedly promi- 

 nent palpebral lobes, the outer arched portion of which, as usual, is con- 

 nected with the side lobes of the first lobe of the glabella. The posterior 

 edge of the cranidium is provided with strong, intergenal spines, which like 

 the ribs run in to the axis. The intergenal spines are situated nearer the 

 occipital segment than the genal spines. Between the intergenal spines 

 and the long, narrow receding genal spines, the posterior edge of the 

 cranidium bends forwards in an arch. The hypostome is elongated and 

 inserted in somewhat broad hypostome attachments. There are i6 uniformly 

 developed thoracic segments; these are distinguished by comparatively 

 narrow pleurae, that terminate in somewhat long, straight, receding pleural 

 spines; from the 13th thoracic segment the pleurae become more and more 

 arched. The occipital and the first 14 thoracic segments are furnished 

 with strong . thoracic spines, that grow longer and longer posteriorly. 

 Those on the nth— 14th segment are especially long. The two extreme 

 posterior segments are without spines. The pygidium is small, shaped like 

 a patch, with indistinctl}' jointed axis and small rim. 



1 Matthew: The Protolcmis fauna, p. 144. Trans. Roy. Soc. Can. \'ol. XI, p. lao. 



- Compare the cranidiums of Aiicniocare liinbaiimi Ang. and laeve Ang. with Agrau- 



los {== Strenuel/aJ depressa Gronwall (K. A. Gronwai.l, Bornholms Paradoxides lag, 



1912, pi. 41. 



