JOHAN KIÆR. M.-N. Kl. 



As far as concerns the facial suture, Walcott thus appears to assume 

 that in the further development from Mesonacidae to Paradoxidae it has 

 been reformed; this does not appear to be very probable as it would be 

 contrary to one of the most important laws enunciated by Dollo for evolu- 

 tion: L'évolution est discontinue, irreversible, limitée. It was 

 this that first made me inclined to doubt whether Walcott really was 

 correct in his views. On closer investigation of the two families it was found 

 that they differed in a whole series of other characters, several of which 

 in my opinion are of systematic importance. 



First of all, we must consider the structure of the glabella and 

 the palpebral lobe. It must be admitted that the form of the glabella 

 in several of the Mesonacidae, e. g. Holmia and Wanneria strongly reminds 

 one of Paradoxides-(orms, although in the latter it is always broader. 

 Nevertheless this resemblance is rather superficial and does not embrace all 

 important characters. Whilst in the former none ^ of the furrows of the 

 glabella continue uninterruptedly across the glabella, in Paradoxides, besides 

 the occipital furrow we find one and most often two of the hindmost furrows 

 going straight across, both in the oldest stages and in the youngest forms^, 

 and it is precisely this that gives the glabella its unique character. More- 

 over, the highly developed palpebral lobes in Mesonacidae, proceed without 

 interruption to the posterior angles of the frontal lobe, whilst as is known 

 this is never the case with Paradoxides. It is this latter case in particular 

 that must be regarded as important, since it is constant and no transition 

 can be demonstrated. 



The probable course of the Facial Sutures is described in a special 

 section in the case of Holmia and Kjerulfia (page 8i). It differs from the 

 same feature in Paradoxides, particularly by the circumstance that the anterior 

 branch is even more markedly deflected. It is conceivable, however, that 

 originally this varied in Mesonacidae. 



On the other hand the Hypostome does not differ greatly in its 

 structure. In this respect Holmia and even more so Callavia and Kjerulfia 

 entirely agree with certain Paradoxides-forms, and we cannot place very great 

 importance upon the marginal spines, which vary strongly in both families. 



It is interesting to observe that the hypostome in most of the Pcwa- 

 doxides species is immovably fixed to the doublure, in the same manner as 



' But possibly in Nevadia; the specimens, however, appear to be greatly compressed, so 

 that thereby the original condition is not prominent. 



2 Only in Par. Hicksii Salt, and Sjögreiii Lnrs., the furrows do not appear to run 

 straight across. 



