Fishes of the Western North Atlantic 3 5 



2 15, fig. 158) and Roule i^2: pi. 4, fig. 7) have presented drawings of young A. sturio 

 in which the lower lobe is not well grown.] 



Head length is the distance from the anterior tip of the snout to the rearmost 

 point of the opercle, opercular membrane excluded. Snout length is from the ante- 

 rior tip of the snout to the anterior margin of the orbit, the membranous rim included. 

 Mouth width is the greatest transverse distance across the mouth slit, with the thick 

 lips excluded and the mouth closed. Postorbital distance is that from the posterior 

 edge of the orbit, membranous rim included, to the rearmost point of the opercle, 

 membrane excluded. Interorbital width is the minimum distance across the top of 

 the head between the bony edges of the orbits. Gill rakers are counted on the outer 

 surface of the first branchial arch, including the rudimentary ones; they are usually 

 counted on the left side of the fish; in specimens smaller than eight inches (20 cm), 

 some rakers on the upper limb are not yet developed; therefore the counts for young 

 individuals are not given (see also 84.'. 149—154). 



Base of anal fin is the length from the structural base of the first ray to the 

 point where the membrane behind the last ray contacts the body. Fin ray counts 

 for the dorsal and anal are sometimes given, but it is difficult to count these accurately, 

 even after staining with alizarin and clearing in glycerin {83: 3-9); the fin membrane 

 is very thick, and the rays are numerous, close together, and show little bifurcation; 

 and in young individuals several rays are detached from the basal elements (radials). 

 Since the counting of rays is not only tedious but uncertain, we omit the number of 

 fin rays from the taxonomic characters. 



Remarks on Identification. Within the same species, various characters change con- 

 siderably with growth; young have much longer snouts than adults, and their scutes 

 (shields) are sharper and closer together. Because of these natural changes it is difficult 

 to prepare a universal key for the specific identification of young specimens as well as 

 adults. There are, it is true, several fairly constant interspecific differences in anatomical 

 features, such as the intestinal tract, gill arches, grinding ridges within buccal cavity, 

 and otoliths, to mention a few. But to present these in abbreviated form seems likely 

 to prove confusing rather than helpful, hence they are not included in the following 

 Key to Species. 



Key to American Atlantic and Pacific Species of Acipenser 



I a. Species of the Atlantic Seaboard. 



2a. Mouth width less than 55 "/o of interorbital; average difference between TL 

 and FL i4''/o of FL; gill rakers 17—27 (av. 21.6); postdorsal and preanal 

 shields in pairs; viscera pale, unpigmented. 



3a. In young specimens 50-70 cm long, head length 26-28 7o of FL; bony 

 shields of dorsal row oval in shape, their longitudinal length being greater 

 than their transverse width; carina on dorsal shields low, without a 

 pronounced hook; dermal ossifications between dorsal and lateral rows 



3' 



