436 PEOCEEDINGS OF THE NATION^AL MUSEUM vol.83 



Genus DIASTYLOPSIS Smith 



DIASTYLOPSIS TENUIS, new species 



Figure 38 



This new species is so extraordinarily closely related to Diastylopsis 

 daicsoni S. I. Smith that I shall limit the description in the main 

 to a differential diagnosis. The body, especially the abdominal por- 

 tion, is slenderer. The subrostral notch is indeed distinct, yet not 

 so deep, owing to the fact that the subrostral angle is not so pro- 

 duced as to form a definite tooth. It is only denticulate, like the 

 anterior margin of the carapace behind it. The oblique lines on 

 the carapace and frontal lobe are present, but only the first line 

 on the carapace is well developed throughout. The others are very 

 weak and often scarcely or not at all perceptible. The layout of 

 the lines also is somewhat different. The first line extends laterally 

 somewhat nearer the margin of the carapace than indicated in 

 Caiman's drawing of dawsoni. The second line has the same posi- 

 tion as in datvsoni but forms a continuation of the first line of the 

 frontal lobe. This, therefore, runs farther forward than in dawsoni. 

 The second line of the frontal lobe lies about in the position of the 

 first in dawsoni, and therefore not on the hinder end of the frontal 

 lobe. The third carapace-line extends in the direction of the pos- 

 terior angle of the frontal lobe and thus lies more posteriorly than 

 in dawsoni. 



The ventral portion of the second free thoracic segment is con- 

 siderably longer; anterior and posterior margins (exclusive of the 

 articular membranes) are approximately parallel, whereas in daw- 

 soni they distinctly converge ventrally. The notch in the anterior 

 lateral margin of this somite is narrower than in dawsoni. 



On the penultimate thoracic sternite of daivsoni there is a single 

 tooth; on the last there are two teeth close together; there is also 

 one tooth on the first abdominal sternite. Our new species has no 

 teeth on the penultimate thoracic and the first abdominal sternite; 

 only the last thoracic sternite bears an unpaired tooth. The pos- 

 terior lateral margins of the abdominal somites are armed with a 

 few isolated denticles. 



The telson is slenderer than in dawsoni. Its length is about 

 three-fourths of the last abdominal somite, which itself also is much 

 slenderer than in daivsoni. Besides the two terminal spines, the 

 telson has only two or three pairs of lateral spines; in daiosoni the 

 ninnber is greater. 



The third maxilliped, as in dawsoni, shows a widening of the 

 basis distally. On the other hand, our species has only one weak 

 denticle on the inner end of the basis, not a strong projecting tooth. 

 The ischium does project outwardly, ending bluntly, but not actually 



