82 JOHAN KIÆR. M.-N. Kl. 



Unfortunately, we have as yet no exact views as to the relation of 

 the other genera of Mcsonacidac in this respect. 



Several of them, if not all, appear to have hypostome attachments. 

 Most often, to judge by the figures, these are quite narrow as in Nevadia{?), 

 Mcsonacisi^), Eliptocephala, Pacdcnmias and Olemlliis. Less often they 

 are broad as in Callavia and Wawicria, which in this respect remind 

 one of Holniia and Kjeriilfia. 



It is still less certain how the hypostome is attached to the hypostome 

 attachments. The condition in Holmia and in Kjerulfia does not appear 

 to be the common one. Thus we know that in Paedciimias transitans it 

 is fastened to the long, narrow hypostome attachments by a narrow medial 

 process (support). Presumably the same is the case in the species within the 

 genus Olenellus, which must be assumed to have developed from Paedemnias. 



On the other hand, to judge from illustrations, the hypostome in Calla- 

 via appears to be loosely attached to the hypostome attachment. 



It would be of great interest to clear up these conditions in the 

 various genera. An investigation of that kind would probably throw light 

 too on the phylogenetic development. 



Probably the hypostome attachment reoccurs as what is called the 

 rostrum in a series of later trilobite families. With respect to the develop- 

 ment of this interesting portion of the shell we refer to Barrande's standard 

 work on the Bohemian trilobites, pi. I — III. 



The rostrum is bounded by three sutures: in front by the rostral 

 suture, at the sides by the short connecting sutures, and behind towards 

 the hypostome by the hypostomal suture. It may vary greatly in form 

 and size, but always serves as an attachment for the hypostome. 



Apparently it is quite lacking in these families in which the facial 

 suture does not cut the anterior margin, but unites in front of the glabella 

 on the upper side of the cranidium, e. g. in Asaphidac, Phacopidae, etc. 

 It is difficult to decide whether the whole portion of the shell at this point 

 has disappeared by a gradual reduction in development, or is only appa- 

 rently lacking on account of a complete growing together of the sutures. 

 The medial suture in AsapJiidae might well support the former hypothesis. 



In Paradoxidac, which in many respects reminds one af Mesonacidac 

 in structure, thé rostrum is again wanting. In this family we are probably 

 correct in assuming that it originally had a form resembling that of 

 Mesonacidae, but the sutures have grown together completely with the 

 doublure, and thus they only apparently disappeared. 



^ Walcott, Olenellus and other Genera of the Mesonacidae. PI. 34, figs. 6—7. 



