42 The Ottawa Naturalist. [June- July 



Development and Distribution. 



The writer does not propose here to discuss the genus 

 Plethopeltis but there are one or two points which are of interest 

 regarding the morphological development and migration of the 

 species, P. saratogensis. By glancing at the diagrams it will 

 be seen that both 1 and 2 are referred to the same species, 

 although certain morphological features are shown to be more 

 strongly developed in one than in the other. As has been men- 

 tioned previously, the " smooth-glabella " forms predominate 

 in the present collection and it is reasonable to suppose that this 

 form is also the more stable, exhibiting more specialized develop- 

 ment. Why the type possessing glabellar furrows and ridge 

 should have persisted may be attributed to some inhibitor which 

 is difficult to explain at present. At first the writer was led to 

 beHeve that the differences of glabellar furrows and circum- 

 glabellar furrow was one mainly of preservation, but a more 

 careftd inspection of the material has led to the conclusion that 

 this is not the case and that we have in the specimens collected 

 from the Hoyt Quarries two distinct types, showing stages of 

 gradation from the smooth to the furrowed form. In the 

 development of the species the glabellar ftirrows and ridges are 

 the first to disappear while the circum-glabellar furrow often 

 persists into the more specialized individual. 



Gushing and Rudemann (9) describe the rocks in which the 

 species occur as follows: 



" the Hoyt is a local phase of the upper Theresa, 



probably an off-shore phase . The waters were clearer, 



less subject to incursions of sand, Crytozoon reefs flourished as 

 they did not in the normal Theresa, and trilobites and gastro- 

 pods lived on the surface of the reefs, where we find their fossil 

 remains to-day." 



When we consider the specimens of .4. saratogensis de- 

 scribed by Weller (6) from New Jersey we notice here that only 

 the "smooth glabella" forms are represented. Weller stated 



that : " glabellar furrows are wholly 



absent from the New Jersey specimens." The pygidia associated 

 with the New Jersey specimens do not entirely agree with the 

 description of that portiom of the animal as it occurs at Saratoga, 

 the transverse furrows being much less conspicuous. Notwith- 

 standing these differences the specific indentity of the specimens 

 from these two localities can hardly be questioned. Most of 

 the specimens observed are smaller than the one illustrated, 

 some of them being less than 5 m/m. in length. The writer also 

 found a large number of small individuals amongst the specimens 

 from the Hoyt Quarry, but these were not measured for obvious 



