878 C. E. Beecher — Larval Form of Triarthrus. 



of hornblende crystals in stringy forms mixed with masses of 

 fine crystal particles of very low double refraction and high 

 single refraction and which appear like masses of zoisite. 

 Still farther southward this character of the rock together 

 with the amount of hornblende increases. A somewhat similar 

 form is found on Bull's Point and in addition some triclinic 

 feldspar is present. It is indeed possible, as suggested by Dr. 

 Foerste, that some former basic eruptive rock was present and 

 included in the metamorphism produced by the granite, 

 although in this case it could have formed but a relatively 

 small proportion of the triangular altered area and its charac- 

 ters are now too much obscured for recognition. On the other 

 hand the presence of feldspar as a result of contact meta- 

 morphism is well known especially in such rocks as contain a 

 considerable amount of alkalies as suggested by Harker.* 



Mineralogical - Petrographical Laboratory, 

 Sheffield Scientific School, New Haven, June, 1893. 



Aet. LII. — A Larval Form of Triarthrus / by 

 C. E. Beecher. 



In the August number of this Journal, Mr. W. D. Matthew 

 made the important announcement of the discovery of 

 antennae and other appendages of Triarthrus found by Mr. W. 

 S. Yaliant near Borne, N. Y. The Yale Museum has received 

 considerable material of a similar nature, which will doubtless 

 furnish additional details of the structure of this trilobite. In 

 the preliminary examination of this material, a number of 

 larval specimens have been discovered, and several other points 

 of interest observed, a few of which are here noticed. 



A description and illustration of the nearly complete meta- 

 morphoses of Triarthrus Becki was given by Walcott in 1879, f 

 based upon specimens from near Holland Patent, ~N. Y. The 

 earliest stage then known in this genus consisted of indi- 

 viduals having the cephalon and pygidium defined and one 

 thoracic segment. With the new material, it is now possible 

 to add an earlier stage in which the thorax is undeveloped and 

 the cephalon predominates, while the other parts are not 

 clearly differentiated. This stage corresponds to the earliest 

 yet discovered in other genera ; as in Sao, Ftychojparia, and 

 Acidaspis. 



The larval Triarthrus at this period is ovate in outline, 

 widest behind, where it also attains its greatest convexity. 



* Quar. Jour. Geol. Soc, Aug., 1891, vol. xlvii. 

 f Transactions of the Albany Institute, vol. s. 



