156 PALEONTOLOGY OF NEW YORK. 



A most remarkable feature of this genus is the variability in the position of 

 the loop. This is not a specific character, but a matter of variation among 

 individuals of a given species. For example, in the type-species, Z. modesta, 

 the loop may be found in any of the four positions represented in the accom- 

 panying figures, which have been made from actual preparations. The same 

 peculiarity is shown in Z. Cindnnatiensis, Meek, Z Kentuckiensis, James, and 

 Z. recurvirostra, Hall. 



It is, nevertheless, true that, of a given number of individuals, say of 

 Z. modesta, the larger fraction will have the loop near the posterior limb of the 

 primary lamellae ; at the same time, examples thoroughly typical in all other 

 respects, will have this organ placed as far forward as it ever occurs in the 

 species Z. recurvirostra, while in the latter species the larger percentage of a 

 given number of individuals will have the loop anterior, and a few have it 

 situated medially. These two species, Z. modesta and Z. recurvirostra, represent 

 the extreme possibilities of variation in this respect, and while it may be said 

 that the normal position of the loop in the former is posterior, and in the latter 

 anterior, yet the variations of the one anteriorly, pass the limit reached by the 

 variation of the other posteriorly. This mobility in the loop of Zygospira is 

 without parallel among other genera, and it has led to some erroneous observa- 

 tions and determinations, based upon insufficient data.* 



The term Anazyga, therefore, which was proposed by Davidson in 1882f 

 for the species Atrypa recurvirostra. Hall, on the basis of its anterior and 

 recurved loop, must be rejected. In the illustration which this author has 

 given, the position of the loop is, perhaps, more extremely anterior than in 

 any of the numerous preparations we have studied, but there can be no 

 question that characteristic examples of the species have a condition of 

 the brachial apparatus which is indistinguishable from that of Z. modesta. 



Ztgospiba has a very considerable representation in species. It seems to 

 have made its appearance in the fauna of the Trenton limestone, in Producta 



* Refei-enc« is made to the observations and criticisms by Dr. Davidson in tlie Supplement to the 

 British Silurian Brachiojioda (p. 122), upon the original determination and illustrations of the position of 

 the loop in Z. inodesta. The latter are correct, though they may not represent the extreme posterior 

 position assumed at times by the loop of^this species.*] 



t Op. cit., p. 128, , 



