﻿NORTH AJMERICAN NOTOSTRACA — LINDER 5 



supra-anal plate. I measure the length of this plate from the most 

 anterior point of the basis of the caudal filaments to its apex. 



As Barnard (1929) and S0mme (1934) have pointed out, the total 

 length of an animal cannot be measured with proper accuracy in pre- 

 served material. The reason is simple enough. Anteriorly and pos- 

 teriorly, the integument of a ring is much softer than in the central 

 part. This soft integument may form a fold directed inward and for- 

 ward, and in such cases the rings appear short and the total length 

 of the animal is relatively short, too. But other specimens from the 

 same lot, which have a carapace of similar length, may show no deep 

 fold in these places, the soft integument being more or less stretched 

 out between the more highly chitinized parts of the rings (fig. 1). In 



J^V ^v^ 



Figure 1. — Schematic diagram of the body-rings of a notostracan showing how the length 

 is influenced by varying contraction. Highly chitinized parts of the integument indi- 

 cated by heavy lines, a, Contracted; h, extended. 



the latter case, of course, the rings appear longer and the total length 

 of the animal may be considerably greater than in the former cases. 

 It is quite usual to meet with both extremes and a more or less con- 

 tinuous series of intermediate cases in a lot containing a large number 

 of specimens, as they may have been fixed in varying stages of con- 

 traction. Rosenberg (1947) noticed a considerable shrinking of the 

 specimens when the}^ were placed in preservation fluid. It is also 

 obvious that preservation, after an indeterminable time, weakens 

 the soft tissue in and between the body-rings and thus causes a length- 

 ening of the body to a variable degree. In very old material macera- 

 tion (and lengthening) may have gone rather far. It is, however, 

 desu-able to give an idea of the length even if it cannot be accurately 

 stated. I measure the length on the midhne from the apex of the 

 head to the base of the caudal filaments. Thus, in Lepidurus, the 

 length of the supra-anal plate is not included in the figm"e of the total 

 length. This may seem a little odd, but in this way comparable 

 figm-es for Lepidurus and Apus are obtained. The length of the supra- 

 anal plate in the former genus may be up to 44 percent of the length 

 of the carapace, and thus it would be quite misleading to include this 

 plate, which has no counterpart within the genus Apus. 



