56 JOHAN KIÆR. M.-N. Kl. 



This interesting phenomenon is also seen in quite the same manner in 

 Kjendfia lata, in which form on account of its considerable size the struc- 

 ture is more easily observed and will consequently' be more closely described. 



Ontogenetic d e \- e 1 o p m e n t. 



I have succeeded in finding quite young stages and other early 

 stages of growth of this form. Only a few cranidiums have been found 

 and they do not give such a complete picture of the development, as the 

 numerous stages known in the case of certain American Mcsotmcidae; 

 nevertheless they are of considerable interest. 



In my description of these stages of growth I propose to adopt Mat- 

 thew's nomenclature and thus differentiate the following stages: 



I. Protaspis stages. {^^ protoiiaHplii(s). We find several stages, several 



dermal castings taking place before the segments begin to form. 

 II. Nepioiiic stages. The cephalon and pygidium are distinct, but the 

 thorax is incomplete. There are just as many nepionic stages as there 

 are thoracic segments. 



III. Neanic stages. All parts complete, but "average growth" incomplete. 



IV. Ephebic stages. The fully developed stages. 

 V. Gcrontic stage. The stage of old age. 



There occur two stages of protaspis, the most primitive of which is 

 shown in pi. VI, fig. i. 



This specimen, which is 1.25 mm. long, was found, in the Ho/im'a 

 shales at Tomten by Amanuensis J. Schetelig some 3'ears ago. In front 

 we see a broad, flat brim, which bends in at the side towards the palpe- 

 bral rib. In the axis 4 distinct segments are clearl}' shown, the foremost 

 lobe is not N'et differentiated from the palpebral ribs which extend right 

 back to the posterior margin in large arcs. Inside the said ribs we see a distinct 

 segmentation of the cheeks. The last segment but one forms a distinct 

 intergenal rib. which extends out into marked intergenal spines. This stage 

 corresponds to an early protaspis stage (paraprotaspis). Cf a correspon- 

 ding one in Elliptocephala asaphoides Emmons, (fig. 9, pi. 25, in Walcott, 

 Olenellns etc., 1910). 



Another stage of almost the same size was at one time found by 

 Professor Brøgger in the Holmia shale at Kletten. In this the frontal lobe 

 is separated from the palpebral ribs. It is thus somewhat more advanced. 



A farther advanced stage of growth is represented by the little crani- 

 dium which is depicted in pi. VIII, fig. 2. The specimen was found by the 



