264 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



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 sis 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 (cf. Table of 

 Monoglyphic Type), it is clear that in the monoglyphic type there are 

 three structural subtypes characterized respectively by five, 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 diglyphic and the monoglyphic type ; for the 

 monoglyphic has obviously a greater range in variation, as shown in 

 its three subtypes, than the diglyphic with only a single one. It is an 

 interesting fact in this connection, that the monoglyphic subtype with 

 six pairs of non-directives often repeats (Fig. -i), so far as its com- 

 plete mesenteries are concerned, the arrangement of mesenteries found 

 in Scytophorus, for which R. Hertwig ('82, p. 104) constructed a sep- 

 arate family, the Monaulese. 



It might at first be suspected that the three monoglyphic 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 monoglyphic type with six pairs of non-directives 

 (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 mesenteries. 

 Such cases as these are so fi-equent, 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 



