264 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
one would expect the monoglyphic type with five pairs of directives to 
be most often met with. Such, however, is not the case, for specimens 
with six or seven pairs of non-directives are about as numerous as those 
with five. Since any of the three groups with five, six, or seven pairs 
of non-directives is represented by a greater number of individuals than 
all the other minor groups of variations taken collectively (of. Table of 
Monoglyphic Type), it is clear that in the monoglyphic type there are 
three structural subtypes characterized respeotively by fivo, six, and 
seven pairs of non-directives, instead of only a single such subtype, as in 
the diglyphic condition. These relations indicate a certain degree of 
distinctness between the diglyphie and the monoglyphie type; for the 
monoglyphic has obviously a greater range in variation, as shown in 
its three subtypes, than the diglyphie with only a single one. It is an 
interesting,fact in this connection, that the monoglyphie subtype with 
six pairs of non-directives often repeats (Fig. 4), so far as its com- 
plete mesenteries are concerned, the arrangement of mesenteries found 
in Seytophorus, for which R. Hertwig ('82, p. 104) constructed a sep- 
arate family, the Monaulec. 
It might at first be suspected that the three monoglyphie subtypes 
pointed out above, and in fact all the variations in the number of com- 
plete mesenteries, could be explained on the assumption that certain 
incomplete mesenteries by excessive growth had become complete, or 
that complete ones had become incomplete, thus introducing a varia- 
tion in the number of complete mesenteries, without, however, altering 
the total number of all kinds of mesenteries ; but in the individuals ex- 
amined the relative development of the incomplete mesenteries was 
found to be subject to so much variation that the satisfactory deter- 
mination of the total number of mesenteries as a basis of comparison 
was practically impossible, and all attempts to carry through interpre- 
tations such as that suggested above resulted in such ambiguous and 
strained results that the unnaturalness of the method condemned it. 
Moreover, in the monoglyphie type with sé pairs of non-direetives 
(Fig. 4), incomplete as well as complete mesenteries are sometimes so 
symmetrically placed that no attempt to readjust them is warranted. 
What may be said of such cases is, that, in place of the usual five pairs 
of non-directives, six pairs are present, and this increase cannot be 
ascribed to reinforcement from the ranks of incomplete mesenterics. 
Such cases as these are so frequent, and instances that may be inter- 
preted as the conversion of complete into incomplete mesenteries or 
the reverse are so few, that it must be admitted, I believe, that these 
