Key for subfossil Heterotrissocladius*

1. • a) Gular region behind setae submenti darkly pigmented (Fig. 1, 2)
     .............................................................................2.
     
     
   • b) Gular region behind setae submenti lightly pigmented (Fig. 3)
     .............................................................................3.
   
   
   
   
2. • a) Mentum with an odd number of teeth; median tooth separated from first lateral tooth by a broad notch, angle at base of notch greater than or equal to 45 degrees (Fig. 1)
     .............................................................................H. maeari group
     
     
   • b) Mentum with an even number of teeth; median tooth separated from first lateral tooth by a narrower notch, angle at base of notch about 30 degrees (Fig. 2)
     .............................................................................H. marcidus group
     
   
   
   
3. • a) Median teeth separated from first lateral teeth by a pair of small accessory teeth (in worn specimens the accessory teeth may not be apparent, but distinct shoulders should remain at the base of the median teeth)
     .............................................................................H. subpilosus group
     
     
   • b) Median teeth of mentum simple, with neither an accessory tooth nor a distinct shoulder(Fig. 3)
     .............................................................................H. grimshawi group
     
     
     

Fig. 1. Heterotrissocladius maeari group. Note the single median tooth. Although this character will not be evident in split head capsules, the distinct notch between the median and first lateral teeth may also provide means for distinguishing this taxon.

Fig. 2. Heterotrissocladius marcidus group. Note that the gular region is uniformly dark, from the mentum to the posterior margin of the head capsule.

Fig. 3. Heterotrissocladius grimshawi group. Note that the region behind the setae submenti, extending to the posterior margin of the head capsule, is lightly pigmented. The median teeth have neither accessory teeth nor low shoulders.