268 C. E. Beech er — Origin and Significance of Spines. 



The differentiation of Achatmella has already been discussed 

 (p. 132) as affording a striking instance of free variation 

 among the Mollusca. The evolution of the Tertiary species of 

 Planorhis at Steinheim, as described by Byatt," furnishes 

 another example, though in neither case has the differentiation 

 of structures proceeded far enough to result in spines. The 

 costate form (Planorbis costatus) was tending toward that end, 

 but did not attain it. 



The series of Slavonian Palndina in the Lower Pliocene, 

 as elucidated by Neumayr and Paul, 50 show a somewhat further 

 advancement. The species in the lowest beds (typus Paludina 

 Neumayri) are smooth and unornamented. Higher in the 

 strata, they are angular and carinated, and at the top of the 

 series, the shells are carinated, nodose, and subspinose (typus 

 Paludina' Hwmesi). The living American genus Tulotoma is 

 closely related to the most differentiated species (P. II<irnesi\ 

 and its approach to spinose features is more pronounced. 



Under the phylogeny of spinose forms (pp. 18, 19) an outline 

 of the life history of the Brachiopod Atrypa reticidaris and 

 derived species was presented. This being one of the com- 

 monest types of Brachiopods in the Silurian and Devonian, 

 often forming beds of considerable extent, it seems quite 

 likely that its prolonged development under favorable condi- 

 tions for multiplication must have had an effect on the amount 

 and kind of variation. 



It has been noticed by Brady 9 and others, that in the Forami- 

 nifera, Globigerina bulloides, Orbulina universa, etc., the 

 pelagic forms comprise two varieties which are generally dis- 

 tinct, a spinous form and another with small minutely granular 

 shells. The bottom specimens of the same species are also 

 commonly without spines and often smaller. The interpreta- 

 tion seems to be that the large specimens indicate an abundance 

 of nutrition which has also produced hypertrophy of the 

 normal granules into spines. Some bottom specimens are 

 large, but they are usually abnormal and of a monstrous or 

 pathologic nature. 



From the foregoing examples, the conclusion to be drawn is 

 that, with full nutrition, there comes a numerical maximum, 

 and naturally with this a corresponding number of normal 

 variations. Some of these modifications, as spines, have arisen 

 by hypertrophy. After having thus originated by growth 

 force, they may or may not be of use for offense, defense, or 

 concealment, or in any way give their possessor a distinct 

 advantage. 



[To be continued.] 



