1870.] 33 [Hyatt. 



D'Orbigny's figures, differ more from Birchii than they do from Amm. 

 muticus, a true armatoid species, though I think this difference, per- 

 haps, is less than it appears to be from D'Orbigny's figures. M. 

 Birchii has two series of forms, as is usual among the Ammonites, 

 one a thick gibbous form, and the other thinner. Brevispina is 

 therefore a different species, a stunted or dwarf descendant of 

 Birchii. 



The young of M. Birchii are round, smooth shells, like Thysanoce- 

 ras Jimbriatum, marked by prominent lines of growth which represent 

 transient mouths and finally pilae. They increase very gradually in 

 size, and acquire a line of genicular tubercles on the fourth whorl, 

 which augment rapidly in size and prominence. On the first quarter 

 of the fifth or last of the fourth whorl an internal line of tubercles 

 appears. These increase very slowly in prominence, until they equal 

 those of the outside line. Occasionally the pilse become bifurcated, 

 and sometimes they cross the abdomen, producing a very faint resem- 

 blance to planicosta. This last, however, is very faintly and very 

 seldom expressed, and then at a comparatively late period of the 

 growth, so that Birchii cannot be said to closely resemble D. Du- 

 dressieri in this respect. 



The septa on the first quarter of the third whorl acquire three mi- 

 nor cells on superior lateral cells, and the superior lateral lobes be- 

 come divided by the rise of a minor cell from the side of the superior 

 lateral cell. On the last quarter of the fourth whorl these have al- 

 ready become equally divided by the increase of this cell, and the 

 lobes and cells possess much of the adult complication, though the 

 lobes are no deeper than the abdominal lobe. From this it may be 

 seen that the progress in complication is very rapid. Since on the 

 third whorl even the septa have already become nearly as compli- 

 cated as those of planicosta at a very much later period, and on the 

 fourth whorl are very similar to those of D. Dudressieri on the sixth 

 whorl. And on the early part of the fourth whorl, when the superior 

 laterals become equally divided, they must be very similar to those 

 of the adult M. biferum, and in fact cannot do otherwise than closely 

 resemble them. Sometimes the young have broad tubercles with 

 the pilse split into two or three parts as in subarmatus, etc. 



Microderoceras Hebertii. 



Amm. brevispina D'Orb., Terr. Jurass., Ceph. p. 272, pi. 79. 

 " Hebertii Opp., der Jura, p. 278. 



PROCEEDINGS B. S. N. H.— VOX,. XIV. 3 JUNE, 1871. 



