THE MOUNT DESERT REGION 7 



Tracliydemidae 



Trachydemus Zelinka 



Tracliydemiis mainensis spec. nov. (figs. 4 to 6) 



Tlie only previously known species of Trachydemus is 

 T. giganteus Zelinka. From lack of specimens, I have not 

 been able to compare the new form with the larval species 

 grouped in Leptodemus. Except for the lack of cereal spines, 

 T. mainensis bears a most striking resemblance to Pycnophyes 

 kielensis. 



Length: 0.61 to 0.63 mm.; width: 0.19 mm. Length-width 

 ratio, about 3.2 : 1. 



There are no middorsal spines. The body is noticeably 

 contracted at the ends, the width of the posterior margin of 

 the twelfth zonite being only slightly more than half that of 

 the middle zonites. 



The anterior dorsal margin of zonite III is evenly rounded. 

 There is no dorsal pachycycle. The midventral plate has its 

 anterior margin straight, the plate therefore projecting above 

 the ventrolateral plates at the lateral corners. The ventral 

 pachycycle of zonite IV is well developed, covering nearly 

 one-third the area of the zonite. 



The posterior extremity of the male is rounded and pro- 

 vided medially with a row of fine hairs. 



Habitat : A mud flat near low-tide line. 



Cotypes: G 1, two males (Collection Biological Survey 

 Mount Desert Region). 



Remarks: This form is shorter and broader than T. 

 giganteus. It also lacks middorsal spines. The two points 

 or spines near the lateral margins of the last zonite, which 

 Zelinka refers to as the lateral angles of this zonite, appear 

 to be greatly reduced cereal spines. They are bifid or trifid 

 in giganteus and club-shaped in mainensis. 



The study of this interesting species lead's me to feel that 

 eventually the distinction between the Tracliydemidae and 

 the Pycnophyidae will disappear. It seems very probable 

 that forms will be found with short but evident cereal spines 

 and also forms in which these spines are long, but clavate or 

 multifid. 



