8(58 



.SCANDINAVIAN I'ISHES. 



260 — 270 111111. tlic abin-e-inontioncd rctrogri'ssioii lie- 

 giri.s to assert itself in tlie clumgos of growth. The 

 four age-classes max tliercfore be named: iiors, Mor- 

 vors, f^lo»i, and stdrsloiii. < hily in tiie tliird of these 

 cla.sses have we been able to examine fully comparable 

 specimens of all the forms, in order to obtain an ex- 

 pression for the se.xual differences within the genus. 

 One of the most distinct examples of the general 

 tendency shown by the development of form M'ithin 

 the genus — among the many that migiit be adduced" — 

 we find in the averages for the alterations caused by 

 growth in tlie relative length of tlie suboperculum. 

 The said length, expressed in percent of that of the 

 head, and reduced to an average for the whole genus 

 OsDierns, is 28'5. The several averages, as expressions 

 of the form-series and the changes of growth, are the 

 tollowinn-: 



Here the averages decrease with increasing age, 

 and the form-series also shows uniformly sinking aver- 

 ages. No constant sexual difference can be discovered, 

 for the male averages in 0. dentex and 0. spirinchus 

 are less, in 0. ei')erJanus greater, than the female. 



Tiie case is different, M'hen a constant sexual cha- 

 racter exercises its intluence on the development. This 

 appears, for example, in the relative dimensions of the 

 eyes. The vertical diameter of the ej'es, calculated on 

 an average for the whole genus Osmerus, is 41 '5 % of 

 the length of the inaxillaries. The se\eral averages 

 ex])ressing the difference between tiie forms and the 

 changes of growth, are the following: 



Here too the changes of growth show sinking aver- 

 ages; Init in the form-series Osmerus spiriiiclii/s occu- 

 pies the most advanced rank, as the representative of 

 a fairly distinct female character. Should we desire 

 to see how the relation appears when this form breaks 

 the series b}' too high an average, ^\■e have a clear 

 example in the relative length of the lo^\■er jaw. This 

 length, expressed in percent of the length of the head 

 reduced, and calculated on an average for tlie whole 

 genus Osmerus, is 91 '9. The averages for the form- 

 series and for the several ages are the followina': 



Here we observe a still more prominent sexual 

 difference both in Osmerus spirinchus and 0. eperlcnitis 

 — and the former together with the females has tlie 

 highest averages — but Osmerus iJeiite.r. the least dif- 

 ferentiated form, shows no sign of a sexual ciiaracter 

 in this respect. 



Again, should we wish to see how specific cha- 

 racters, manifest themselves dui'ing the course of de- 

 velopment, we can Iiardly find a better example tiian 

 among the Smelts. Tlie only characters not infringed 

 by exceptions, i. e. the only constant specific charac- 

 ters — so far as we can gather from our specimens — 

 are the following: 



A: 19 — 20 gill-nikcfs ou the lower part of 

 the first branchial arch, and at most 29 

 on the whole front surface thereof. 

 Height (longest ray) of the anal fin at 

 most 7-() 'o of the length of the body Osmerus dented-. 



B: 23 — 25 gill-rakers on the lower part of 

 the first branchial arch, and at least 

 33 on the whole front surface thereof. 

 a: Height of the anal fin at most 7-6 



°o of the length of the bodv Osmerus spiriiirhus. 



h: Height of the anal fin at least 8-4 



% of the length of the bodv Osmerus epeiiunus. 



The manner in Avhich the height of the anal fin 

 passes through its changes of growth, and affords one 



" Cf. Smitt, Riksmuseets Sulmonidei; pp. 168 — 189. 



