THE ANNULUS OF A MEXICAN CRAYFISH. 1 23 



Of the 179 specimens sent to me 88 were females ranging in 

 length from 16 mm. to 36 mm., many being 25-28 mm. The 

 males ranged from 14 to 36 mm. and many were 25 mm. Saus- 

 sure had 25 specimens, which measured up to 27 mm., while 

 Faxon's largest was 35 mm. That those 30-36 mm. were sexu- 

 ally mature was evident from the fact that six bore eggs, and 

 two, young larvae, attached to the abdominal appendages, as in 

 other crayfishes. 



The general appearance of the annulus in these small, but 

 mature forms is indicated in Fig. 1, which shows the bases of 

 the third to the fifth thoracic legs and the sternal surface between 

 them. While the annulus has the same position as in all other 

 Cambari, between the sterna of the fourth and fifth legs, it differs 

 from any hitherto figured in not being a low transverse plate of 

 more or less annular form but in forming a high transverse papilla 

 of asymmetrical form. 



The sternum between the fifth legs is also more specialized than 

 in higher Cambari in that it projects as a stiff spine, somewhat 

 like a lengthwise ridge, but often more like a conical spine. 



This spine fits into a very marked median groove on the pos- 

 terior side of the annulus in the manner mentioned by Ortmann 

 (2) as characteristic of his new form, Paracambarns paradoxus. 

 The eggs issuing from the elliptical openings on the third legs 

 (Fig. 1) doubtless flow back around the base of this rounded, 

 papilla-like annulus, and there receive the fertilizing sperm, for 

 we find in this annulus a functioning sperm receptacle. 



The appearance of the annulus when cut off and viewed from 

 the posterior side is shown in Fig. 2. The right and left of the 

 high papilla are unevenly balanced about the long deep median 

 groove and on the observer's left, which is the animal's right, there 

 is a peculiar structure, which proves to be the seminal receptacle. 

 The asymmetry of the annulus is marked, the apex being to one 

 side and the side containing the receptacle being abrupt while the 

 opposite one slopes gradually. 



In the crayfish hitherto studied the receptacle is wholly, or in 

 part, on the median line but here we find it entirely to one side. 

 In those crayfish there is, as far as studied, a peculiar dimorphism 

 amongst the females (9), some of them having the entrance of the 



