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. Cincinnatiensis, 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. 
Zygospira has a very considerable representation in species. It seems to 
have made its appearance in the fauna of the Trenton limestone, in Producta 
* Reference is made to the observations and criticisms by Dr. Davidson in the Supplement to the 
British Silurian Brachiopoda (p. 122), upon the original determination and illustrations of the position of 
the loop in Z. 'imdesta. The latter are cori-ect, though they may not represent the extreme posterior 
position assumed at times by the loop of 'this species.’] 
t Op. cit., p. 128, 
